packaging - starting ovirt-engine only after yum rollback

With the previous implementation ovirt-engine service
was started before exiting the rhyum transaction
context after an execption. So the newly installed version
of ovirt-engine was running with the restored PKI and DB.

Now the ovirt-engine service start is done only after
yum rollback transaction.

Change-Id: If8160293d45361534dfda6c6aa3870e72368d10b
Bug-Url: https://bugzilla.redhat.com/879969
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>