mount.glusterfs: Fixed regexp matcher for existing mount points

Change-Id: I58d237a3d2f4caa7f3865c2e4899c472f7457450
BUG: 906887
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/4457
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>