The support for non-keystone auth systems and plugins
was deprecated with 1f11840dd8
back in the 3.1.0 release in mitaka.
Now that we're working on shoring up the support for
keystone auth sessions in the client and eventually moving
to remove support for the non-session HTTPClient, we should
also remove the support to load non-keystone auth plugins.
The whole concept of non-keystone auth systems/plugins is
a legacy artifact and is a barrier to interoperability which
is a goal of nova and OpenStack in general.
Change-Id: Ia649db257c416ca054977812ecb3f1a8044fa584