Change --os-auth-plugin to --os-auth-type

User's don't know what a plugin is.

* Internally, os_auth_type and/or auth_type represents what the
  user supplied.
* auth_plugin_name is the name of the selected plugin
* auth_plugin is the actual plugin object

Plugin selection process:
* if --os-auth-type is supplied:
  * if it matches against an available plugin, done
  * (if it can map to an availble plugin type, done; TODO in a followup)
* if --os-auth-type is not supplied:
  * if --os-url and --os-token are supplied, select 'token_endpoint'
  * if --os-username supplied, select identity_api_version + 'password'
  * if --os-token supplied, select identity_api_version + 'token'

Change-Id: Ice4535214e311ebf924087cf77f6d84d76f5f3ee
Merge "Change --os-auth-plugin to --os-auth-type"