stack preview breaks with multiple environment files

[Steven Hardy]

Until fairly recently, it was possible to preview a tripleo overcloud, but today I see an error similar to this reproducer:

openstack stack create --dry-run foo -t test.yaml -e test_env.yaml -e test_env2.yaml
ERROR: Property error: : resources.nested.properties: : Property hosts not assigned

(undercloud) [stack@undercloud tmp_overcloud]$ cat test.yaml
heat_template_version: ocata
resources:
  nested:
    type: nested.yaml
(undercloud) [stack@undercloud tmp_overcloud]$ cat test_env.yaml
parameter_defaults:
  foo: 123
(undercloud) [stack@undercloud tmp_overcloud]$ cat test_env2.yaml
parameter_defaults:
  hosts: foo

It seems that the second environment file is either ignored or not correctly merged, if you put the "hosts" parameter valie in test_env.yaml, it works.

[Steven Hardy (shardy)]

I think this is a duplicate of https://bugs.launchpad.net/heat/+bug/1669571 - thanks to Rabi for pointing that out