engine: Inform the user when the external scheduler is unavailable

If the external scheduler is configured and not responding,
use the Audit log to inform the user about it.

Change-Id: I5a3e56e9a8db16b58305b2ebdc2b8041297da093
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1001986
Signed-off-by: Martin Sivak <msivak@redhat.com>