Fix usage of admin_endpoint

The admin_endpoint in keystone.conf is used
to advertise for service discovery to a client, and
should not contain the identity version as this will
be appended based on which versions are available.

This patch changes the behavior of the native types that
depend on admin_endpoint and the service validation to
accept a url without a version appended. In both cases,
the version will be set to v2.0, as there is no suitable
conf option in keystone.conf from which the available
versions can be deduced. This allows the correct, versionless
admin_endpoint to be set in keystone.conf for the service
itself.

Change-Id: I0c1ebd9f1cf452828698555e7c642fac770aa73f
Merge "Fix usage of admin_endpoint"