Do not use keystone.conf.sample in tests

The keystone.conf.sample file has no values set in it (anymore)
and therefore should not be loaded by the test cases. The one
exception to this rule is the ConfigTestCase as the config file
is used in one of the code paths to determine paste-ini location.

Change-Id: I6b9dbe160c5ffe26971320a29f933018ac0486c2
Merge "Do not use keystone.conf.sample in tests"