As you already mentioned, this only fails when you try from the GUI. the same templates exactly deploy correctly from the CLI. My CephClientKey in the templates is different from what you see in the output of 'mistral environment-get'. I didn't notice any other keys overwritten, and we also don't have test cases that cover it.

+-------------+---------------------------------------------------------------------+
| Field       | Value                                                               |
+-------------+---------------------------------------------------------------------+
| Name        | extceph                                                             |
| Description | <none>                                                              |
| Variables   | {                                                                   |
|             |     "passwords": {                                                  |
|             |         "CephClusterFSID": "48bfb3fa-f35e-11e6-81f7-525400b8201c",  |
|             |         "CephMonKey": "AQB/GqRYAAAAABAA5Z9ExdTVWPtUxIlf5EJfgw==",   |
|             |         "NeutronPassword": "rtrjmePktB2uCRQjzKctcqbC2",             |
|             |         "HeatPassword": "q2PhCREVMAd8NTrt99ypX2y6z",                |
|             |         "SnmpdReadonlyUserPassword": "c4885c6b71f815068b9617b773a4d |
|             |         "GlancePassword": "sfyYxQQPRwfBaGuY7kEghArfU",              |
|             |         "IronicPassword": "kQW2ZXtGCqxMpamv9DnU9eVhW",              |
|             |         "HeatStackDomainAdminPassword": "rWTvVXh8zKAhByDGyxHVtKxEN" |
|             |         "ZaqarPassword": "K62yBdTgGhveQjDyEJBfYpA8x",               |
|             |         "MysqlClustercheckPassword": "WpdX3Xrrd6uykGFWUpsrWamem",   |
|             |         "GnocchiPassword": "bhQpfxgXtbVq9ywCmsmbbv88T",             |
|             |         "CephAdminKey": "AQB/GqRYAAAAABAAO3vBKwhyY9DdFgrvyq9ybQ==", |
|             |         "CeilometerPassword": "4mHgyUZmJ6TpJUtUnxcYuvYJG",          |
|             |         "CinderPassword": "uhXhpNCCUBwKBMYqH2cMTZ2Y2",              |
|             |         "RabbitPassword": "3UjZqfBtrfjCRaFUDsYK4wEQs",              |
|             |         "CephRgwKey": "AQB/GqRYAAAAABAAEXlbwX+n/Or3JS6nXzPJQg==",   |
|             |         "TrovePassword": "jmHfZ3FE6X8EEY8DTZaj4dhmH",               |
|             |         "AodhPassword": "GZDHu49gqPhEN3pKHVdDvcWFn",                |
|             |         "KeystoneCredential0": "upkK49LbdUixDNjoMTzjhFrwqLiOFcox4Sl |
|             |         "KeystoneCredential1": "9iPRaE5g0INAGDa9JYIDCDjM4T7S-OU4vEs |
|             |         "CephClientKey": "AQB/GqRYAAAAABAAid7+yL6kjBdibN/D+N1qFw==" |
|             |         "NovaPassword": "zdeJDvjt3JEFDqhD4CkkGNfVs",                |
|             |         "AdminToken": "rGcwFtW9W9FRQVqV8gVC8m6K4",                  |
|             |         "RedisPassword": "sGGgeZyMDJ7Z7wmxc3dbpEDdU",               |
|             |         "MistralPassword": "TEBFbGhKk8mMeeunpw22DfQze",             |
|             |         "SwiftHashSuffix": "Vn7MGfzVDCuxYPvEjYpMDsrGn",             |
|             |         "AdminPassword": "4XyZyCMHeuvMsBJXtQ8RRevBu",               |
|             |         "HAProxyStatsPassword": "67Xsjz4AQUnpqtTVb9rEc4FNg",        |
|             |         "ManilaPassword": "h3Wxm6yJ6rTBJwFapWyqC98nj",              |
|             |         "SwiftPassword": "YvMUmQezkqQbH3TqccQjqF4GC",               |
|             |         "CeilometerMeteringSecret": "QHRE8QQXArUPYpjsYWjVFPABQ",    |
|             |         "NeutronMetadataProxySharedSecret": "cBMQH73Pn3nZvPBUC4FtXv |
|             |         "BarbicanPassword": "Z6HZR4qGZ8XYxJ8jJ4Uaaxb7G",            |
|             |         "SaharaPassword": "289GwfG4acWFHxHQrcnMhwhNu"               |
|             |     },                                                              |
|             |     "parameter_defaults": {                                         | 
|             |         "ControllerCount": 3,                                       | 
|             |         "OvercloudControlFlavor": "control",                        | 
|             |         "ComputeCount": 2,                                          | 
|             |         "StackAction": "CREATE",                                    | 
|             |         "OvercloudComputeFlavor": "compute",                        | 
|             |         "node_admin_extra_ssh_keys": "ssh-rsa AAAAB3NzaC1yc2EAAAADA | 
|             | pWNqi65yTJwPGvFuGxNk9aRIDL4diPe8b77Qc7/8I2FLHrxXm3JvKRi2BulaVY+Bc0D | 
|             | m4BUukujPr6xAnXIHEED5UsNcgnBSurF tripleo-validations",              | 
|             |         "DeployIdentifier": 1487149903,                             | 
|             |         "UpdateIdentifier": ""                                      | 
|             |     },                                                              | 
|             |     "template": "overcloud.yaml",                                   | 
|             |     "environments": [                                               | 
|             |         {                                                           | 
|             |             "path": "overcloud-resource-registry-puppet.yaml"       | 
|             |         },                                                          | 
|             |         {                                                           | 
|             |             "path": "environments/puppet-ceph-external.yaml"        | 
|             |         },                                                          | 
|             |         {                                                           | 
|             |             "path": "environments/puppet-pacemaker.yaml"            | 
|             |         }                                                           | 
|             |     ]                                                               | 
|             | }                                                                   | 
| Scope       | private                                                             | 
| Created at  | 2017-02-15 09:08:13                                                 | 
| Updated at  | 2017-02-15 09:11:43                                                 | 
+-------------+---------------------------------------------------------------------+