Unit test for common.utils.cache_from_env
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

smerritt/swift,psachin/swift,wenhuizhang/swift,thiagodasilva/swift,prashanthpai/swift,revoer/keystone-8.0.0,Akanoa/swift,larsbutler/swift,rackerlabs/swift,rackerlabs/swift,nadeemsyed/swift,tipabu/swift,gold3bear/swift,swiftstack/swift,swiftstack/swift,zackmdavis/swift,nadeemsyed/swift,psachin/swift,redhat-openstack/swift,eatbyte/Swift,smerritt/swift,tipabu/swift,aerwin3/swift,matthewoliver/swift,larsbutler/swift,sarvesh-ranjan/swift,hurricanerix/swift,IPVL/swift-kilo,AfonsoFGarcia/swift,mjwtom/swift,notmyname/swift,maginatics/swift,Seagate/swift,shibaniahegde/OpenStak_swift,IPVL/swift-kilo,hbhdytf/mac2,hbhdytf/mac2,thiagodasilva/swift,hbhdytf/mac2,bkolli/swift,Akanoa/swift,takeshineshiro/swift,hurricanerix/swift,smerritt/swift,aerwin3/swift,wenhuizhang/swift,clayg/swift,openstack/swift,Em-Pan/swift,hurricanerix/swift,matthewoliver/swift,hbhdytf/mac,bradleypj823/swift,openstack/swift,dencaval/swift,Em-Pan/swift,hurricanerix/swift,iostackproject/IO-Bandwidth-Differentiation,williamthegrey/swift,daasbank/swift,bkolli/swift,levythu/swift,mjzmjz/swift,openstack/swift,nadeemsyed/swift,smerritt/swift,dpgoetz/swift,daasbank/swift,notmyname/swift,takeshineshiro/swift,scality/ScalitySproxydSwift,Khushbu27/Tutorial,levythu/swift,mjzmjz/swift,nadeemsyed/swift,clayg/swift,redbo/swift,scality/ScalitySproxydSwift,dencaval/swift,sarvesh-ranjan/swift,anishnarang/gswift,bouncestorage/swift,clayg/swift,anishnarang/gswift,gold3bear/swift,dpgoetz/swift,matthewoliver/swift,psachin/swift,tipabu/swift,notmyname/swift,openstack/swift,mjwtom/swift,redbo/swift,williamthegrey/swift,zackmdavis/swift,bradleypj823/swift,tipabu/swift,maginatics/swift,prashanthpai/swift,eatbyte/Swift,hbhdytf/mac2,Khushbu27/Tutorial,clayg/swift,shibaniahegde/OpenStak_swift,psachin/swift,hbhdytf/mac,swiftstack/swift,matthewoliver/swift,notmyname/swift,revoer/keystone-8.0.0,Seagate/swift,AfonsoFGarcia/swift,redhat-openstack/swift,iostackproject/IO-Bandwidth-Differentiation,bouncestorage/swift