fix some weird bugs in lms_auth hash merge conflict

@jarv @feanil @jrbl 

Ansible's variable precedence strikes again.

When the hash merged happened to combine the lms_auth_config in this file and the one in our secrets file, the merge ended up taking data from _both_ sources, in a way I do not understand well.

Let me describe what was being merged.  
The customized settings in our secrets repo had:
```
lms_auth_config:  &lms_auth
  'MODULESTORE':   
    'default':   
      'ENGINE': 'xmodule.modulestore.mongo.MongoModuleStore'
      'OPTIONS':  &lms_modulestore_default_options
          'password':  'OUR REAL PASSWORD'
```
`roles/edxapp/vars/main.yml` used to have
```
edxapp_generic_auth_config:  &edxapp_generic_auth
  'MODULESTORE':
    'default':
      'ENGINE': 'xmodule.modulestore.mongo.DraftMongoModuleStore'
      'OPTIONS':  &generic_modulestore_default_options
         'password':  'password'

lms_auth_config:
  <<: *edxapp_generic_auth
```

When these two hashes merged, we ended up with
```
lms_auth_config:
  'MODULESTORE':
    'default':
      'ENGINE': 'xmodule.modulestore.mongo.DraftMongoModuleStore'
      'OPTIONS':  
         'password':  'OUR REAL PASSWORD'
```
which is just bizzare.

This PR fixes the problem for now, though perhaps at this point we should make explicit override hashes like `lms_auth_override_config` and use jinja2 syntax to do clearer preference...this kind of bug is sort of insidious.