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
c411fcfc92