configfile.py utility for common config file editing operations.

Add utility to edit config files that is backward compatible
with libvirt_configure.sh. Support for common operations used
by vdsm tool. Context managed and commits file upon __exit__
by replacing original file with a temporary file.

Also provide a wrapper around ConfigParser to allow read only
of file with no ini sections([section]).

Change-Id: If8c7ae2b562650e403fc39024f3531d44b2b9c4f
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27129
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>