core: Improving job messages for event subscription commands

Improving job messages for AddEventSubscription and
RemoveEventSubscription

Change-Id: Ic263b8f59c36fc97f1733317edbd17dbaa5487dd
Bug-Url: https://bugzilla.redhat.com/919045
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>