Improve DB API test coverage
Improve DB API test coverage

This patch improves the test coverage of DB API methods a bit.

Blueprint: db-session-cleanup

Change-Id: Ib4b7312b6effca771a80d6b3137005cc28cb81e4

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