logging: Enable all loggers

Unless disable_existsing_loggers is passed as false, logging checks
the pre-existing logs and disables any that are not mentioned in the
configuration file, this causes various logs to be not visible.

Change-Id: Ia6cbf7d749cfc2d432624213bfe5beb26055e5a8
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1060510
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25185
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>