Implement "omni list-users" CLI command

Using the newly-added support for listing user names via
Authenticator.usernames(), implement listing users recognized
by a store/realm in the command line interface.
Use "omni_app" instead of "config" parameter in cmd_authenticate
Add convenience method OMNI.get_realm_or_store()

This is a convenience method to get either a realm or a store from a string.
If the name contains a period ("."), it is assumed to be a store name, else
a realm name. This method is very convenient for implementation of the CLI
commands.