stub cfg.CONF in heat_keystoneclient

It seems that sometimes tests can fail when reading the config
and the suspected cause is a race caused by running the tests in
parallel when using the global set_override on oslo.config.

Stubbing all calls to the config should solve this issue (although
it is a bit cumbersome in the test..)

Change-Id: Ib1ee97f116544e069123253002d4a408503ac82e
Closes-Bug: #1273126
Merge "stub cfg.CONF in heat_keystoneclient"