Keystone clients cannot be configured for internal endpoint

[Andrew Bonney]

We have found in our tests of Magnum+CAPI that the Keystone client code lacks the ability to select anything other than the public endpoint for accessing Keystone.

We are using a hard-coded workaround in our deployment which demonstrates where this is causing an issue: https://github.com/bbc/magnum/commit/b0c67736ccdb17e991cb7006c0664998b64d5c24

For 'neutron_client', 'nova_client' and similar we set the 'endpoint_type' to 'internalURL'. Whilst 'keystone_authtoken' 'interface' is set to the default (internal), this doesn't appear to be referenced by this code path.