Replacing direct import for token plugin

The direct import of the auth plugin
'keystone.auth.plugins.token.Token' is deprecated since
Liberty.

The entrypoint should only be defined in case there is a
need to override the default implementation with a custom
class.

A closer inspection to the code confirms that:
https://opendev.org/openstack/keystone/src/commit/70c9dd825691ce23e644169d5f30045cb21fe442/keystone/conf/auth.py#L38

Closes-Bug: #1837110

Change-Id: I80e584de7d5bf46c5621e357333853975cfebade