ConfigFile.py: provide backward compatibility with '# by vdsm' comments.

see 'Backward compatibility:' at ConfigFile doc for detailed description.

Change-Id: I9546c6a8f127a430aab95a0fc05ee7cb3daa887c
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27822
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>