This patch adds a BaseLoader class for the Kerberos plugin and an entry
point in setup.cfg.
Since the plugin file is being renamed, also fix the comment that
refers to the library as 'keystoneauth' - it is called 'keystoneauth1'
and trying to install 'keystoneauth' will cause the outdated version of
the library to be installed and kerberos will not work.
To make sure the plugin was loadable, this was tested using a version
of python-openstackclient that had been migrated to keystoneauth[1].
[1] https://review.openstack.org/#/c/276350/
Change-Id: Id339295c795f6bf1b428dac8fc9f79d2f5fb453f
Closes-bug: #1567257
Partial-bug: #1567260