Don't use wc -l for the umount check
Don't use wc -l for the umount check

We don't need the follow the pattern of checking wc -l against
/proc/mounts is 0, we care about any match, so we can just use
grep and it's exit code inside the if.

Change-Id: Idd0d78a613f8b43f169a320527c763d0bff74368

openstack/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,takahashinobuyuki/diskimage-builder