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
Do not remove volume silently if GPFS is unmounted

When GPFS is not mounted, delete_volume() will complete successfully, so that
volume record is removed by cinder but volume file still resides on GPFS. This
change will prevent cinder to remove volume when GPFS is not mounted.

Change-Id: Ic3bf265e553f22c8b6a6e3568da04af88fab3322
Closes-Bug: #1249411
Merge "Do not remove volume silently if GPFS is unmounted"
Merge "Report zero capacity if GPFS is unmounted"