Allow installing tempest only with keystone

Modify the lib/tempest to not fail
 if no nova or glance available.

* This allows performance test of keystone and neutron with
  tempest (or tempest stress runner) without having system
  noise from another components.
* Depending on not required components for tempest is bad practice,
  tempest service decorators expected to allow tempest,
  to run in more minimal system out-of-the-box.

Change-Id: Ifc40b1eb5c4b79d96a5fae919b88afecca642ca0
Merge "Allow installing tempest only with keystone"