Unit test for common.utils.cache_from_env

The behavior of common.utils.cache_from_env
was changed by https://review.openstack.org/#/c/89488/.
This patch adds unit test for that function.

Change-Id: If757e12990c971325f7705731ef529a7e2a9eee7
Merge "Unit test for common.utils.cache_from_env"