Unmount the NFS and GlusterFS shares on detach

When attaching an NFS or GlusterFS volume, nova mounts the share so it
can be reused by other instances but it is not unmounted when detaching.

This patch adds the disconnect_volume method to both drivers, which will
try to unmount the share. If a volume in that share is still being used
by another instance, the umount command will fail with the error 'target
is busy' and we will ignore it. Any other error while unmounting will be
properly logged. In any case the detach will fail because nova hasn't
been able to unmount the share.

Closes-Bug: #1285209
Change-Id: I79289524541a88ad359b3465719055cd402b4e3d
Merge "Unmount the NFS and GlusterFS shares on detach"