Description of problem:
When an environment file with an empty resource_registry section is passed to the overcloud deploy command it errors during check_file_for_enabled_service function and returns a trace back


Version-Release number of selected component (if applicable): 
python-tripleoclient-9.2.7-11


How reproducible: 
Every time, easily.


Steps to Reproduce:
1.Create an environment file with the following content:
[stack@undercloud-0 ~]$ cat network-extras.yaml
# ---------------------------------------------------------------------------
# Custom network extras script
# ---------------------------------------------------------------------------
resource_registry:
     # OS::TripleO::NodeExtraConfigPost: extraconfig/post_deploy/default.yaml

2.Include the file in the deploy command with -e

3.Run the overcloud deploy command

Actual results:
2019-10-28 20:59:02.431 18217 ERROR osc_lib.shell [  admin] Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/osc_lib/shell.py", line 134, in run
    ret_val = super(OpenStackShell, self).run(argv)
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 279, in run
    result = self.run_subcommand(remainder)
  File "/usr/lib/python2.7/site-packages/osc_lib/shell.py", line 169, in run_subcommand
    ret_value = super(OpenStackShell, self).run_subcommand(argv)
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 400, in run_subcommand
    result = cmd.run(parsed_args)
  File "/usr/lib/python2.7/site-packages/tripleoclient/command.py", line 25, in run
    super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/osc_lib/command/command.py", line 41, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/cliff/command.py", line 184, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/lib/python2.7/site-packages/tripleoclient/v1/overcloud_update.py", line 77, in take_action
    parsed_args.environment_files)
  File "/usr/lib/python2.7/site-packages/tripleoclient/utils.py", line 1059, in check_deprecated_service_is_enabled
    check_file_for_enabled_service(env_file)
  File "/usr/lib/python2.7/site-packages/tripleoclient/utils.py", line 1042, in check_file_for_enabled_service
    service in content["resource_registry"]):
TypeError: argument of type 'NoneType' is not iterable


Expected results:
If we can't handle files like this then we should return an error rather then a trace back. Something like "The following environment file has an empty resource_registry section: %s"

Additional info: