Report zero capacity if GPFS is unmounted
Report zero capacity if GPFS is unmounted

_get_available_capacity() calls df command to gather the capacity of GPFS
filesystem. If GPFS is unmounted, df command will return the capacity of
root file system. This change will let get_volume_stats() to report zero
capacity, so that Cinder can schedule the volume request to another volume
service After GPFS is mounted again, the correct capacity will be reported.

Change-Id: I4c229d78e7d659604a23a50e6ab46060642b4088
Closes-Bug: #1246779

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