When a non-keystone plugin is used together with an unversioned endpoint,
we give up on discovery before figuring out both major version and
the correct endpoint. This is because get_endpoint_data is called with
discover_versions=False, so discovery assumes we have all information
already. It may be an issue in discovery itself, but I'm afraid to
touch that code. Instead, if get_endpoint_data returns no API version
with discover_versions=False, try with discover_versions=True, which
matches what the identity plugins do.
Also increase the unit test coverage.
Change-Id: Ie623931b150748d7759cf276e0023a2f06a8d4db
981a19bba1