conf: use ovirt-engine-lib configfile

Use ovirt-engine-lib configfile module for reading
configuration files.
The previous implementation had an issue if a variable
was defined recursively in more than one config file.

Change-Id: I1f47875a279953bae783407dca41102904a70061
Bug-Url: http://bugzilla.redhat.com/1022557
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>