services: ovirt-engine: allow all levels within engine.log appenders

this reduces the changes required by user when enabling debug for
specific loggers, reduce confusion and chance for error.

we should have been able to use dynamic log in 3.5 but did not provide
that, so until then make the log enable sequence simpler.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1158757
Change-Id: I4788133905582c80c7ee6275245a07b913759f07
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>