core: adding user message of the day

This patch adds the UserMessageOfTheDay config entry, also allowing to:
1. Set it using engine-config
2. Fetch it in public query, as it will be used in the login page

Change-Id: Ifb207a2505fb2456c21ab48ffc4afefce068e2e2
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=983120
Signed-off-by: Oved Ourfali <oourfali@redhat.com>