Refactor Auth plugin configuration options

The auth plugin configuration can now either be specified by the
[auth] methods and [auth] <method name> options or as a class-name
as part of the [auth] methods option (e.g.
keystone.auth.plugins.password.Password). Both styles of auth
method specification can be used at the same time.  No two auth
plugins may register for the same auth method.

Each auth plugin must now define a 'method' attribute that specifies
the auth method it is registering for.

Tests have been modified to use a unified "method" name, this ensures
we are testing the appropriate code paths for auth methods rather than
relying on strings matching in all cases going forward.

Co-Authored-By: Brant Knudson <bknudson@us.ibm.com>
bp: keystone-parallel-testing
Change-Id: Icbb1d63d36726b8dbf7dfd8c6353fa30bbe490de
Merge "Refactor Auth plugin configuration options"