Move clear_volume back to it's own method
Move clear_volume back to it's own method

This just moves clear_volume back to it's own method
and also only calls volutils.clear if we actually
need to perform the clear operation.

Also takes out the hard-coded multiplier of 1024 and
uses units.GiB.  Maybe this should be a separate patch,
I'll spin another if there are objections.

Change-Id: I56f6690fbadc0d44eba48f49d29386d5b9dca195

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