Falling back to environment variable credentials is convenient for
development but could lead to unexpected credentials being used during
tests. Also this is not a known practice for tempest tests.
This change removes the env var defaults and explicitly sets the
credentials to use in prepare_test_env.sh.
Pre and post grenade tests also get their config from the conf file
rather than the environment.
blueprint tempest-plugin-support
Change-Id: Icf9fb4c58ed11483106c2b62d1e1093223f7dc51
2af6c6ffdc