Sync to newer openstack.common.cfg
Sync to newer openstack.common.cfg

Cherry picks e0d891e from Nova.

Changes since last sync are:

 * New ConfigOpts.find_file() for locating conf files
 * Support for directory source of config files
 * Provide file extension when when looking for files
 * Some refactoring of the cfg cache
 * Add caching to openstack.common.cfg

Change-Id: If9852d0431093d590252bb704e934a4a56ef0263

eharney/cinder,scality/cinder,takeshineshiro/cinder,winndows/cinder,sasukeh/cinder,Thingee/cinder,ntt-sic/cinder,dims/cinder,Nexenta/cinder,rakeshmi/cinder,Hybrid-Cloud/cinder,rakeshmi/cinder,j-griffith/cinder,JioCloud/cinder,julianwang/cinder,Paul-Ezell/cinder-1,julianwang/cinder,maelnor/cinder,sasukeh/cinder,manojhirway/ExistingImagesOnNFS,maelnor/cinder,cloudbau/cinder,tlakshman26/cinder-new-branch,rnirmal/cinder,eharney/cinder,rnirmal/cinder,saeki-masaki/cinder,NewpTone/stacklab-cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,phenoxim/cinder,apporc/cinder,citrix-openstack-build/cinder,spring-week-topos/cinder-week,potsmaster/cinder,inkerra/cinder,redhat-openstack/cinder,NeCTAR-RC/cinder,leilihh/cinder,dims/cinder,cloudbau/cinder,manojhirway/ExistingImagesOnNFS,abusse/cinder,NewpTone/stacklab-cinder,duhzecca/cinder,winndows/cinder,phenoxim/cinder,NewpTone/stacklab-cinder,blueboxgroup/cinder,Nexenta/cinder,freedomhui/cinder,Hybrid-Cloud/cinder,bswartz/cinder,CloudServer/cinder,openstack/cinder,NeCTAR-RC/cinder,Paul-Ezell/cinder-1,rickerc/cinder_audit,saeki-masaki/cinder,Datera/cinder,rickerc/cinder_audit,takeshineshiro/cinder,leilihh/cinder,rnirmal/cinder,petrutlucian94/cinder,Accelerite/cinder,Datera/cinder,potsmaster/cinder,ntt-sic/cinder,nikesh-mahalka/cinder,cloudbase/cinder,nexusriot/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,cloudbase/cinder,citrix-openstack-build/cinder,Akrog/cinder,petrutlucian94/cinder,inkerra/cinder,Thingee/cinder,Thingee/cinder,github-borat/cinder,hguemar/cinder,duhzecca/cinder,alex8866/cinder,freedomhui/cinder,github-borat/cinder,openstack/cinder,j-griffith/cinder,apporc/cinder,freedomhui/cinder,scottdangelo/RemoveVolumeMangerLocks,nexusriot/cinder,mahak/cinder,mahak/cinder,CloudServer/cinder,tlakshman26/cinder-https-changes,hguemar/cinder,scottdangelo/RemoveVolumeMangerLocks,NetApp/cinder,nikesh-mahalka/cinder,bswartz/cinder,blueboxgroup/cinder,ge0rgi/cinder,spring-week-topos/cinder-week,NetApp/cinder,tlakshman26/cinder-https-changes,Accelerite/cinder,alex8866/cinder,citrix-openstack-build/cinder,Akrog/cinder,JioCloud/cinder,scality/cinder,abusse/cinder,redhat-openstack/cinder,tlakshman26/cinder-new-branch