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
Merge "Report zero capacity if GPFS is unmounted"