Notification service should send a notification when Database is down.

Send a notification email after first failure to fetch notifications,
and then once every failedQueriesNotificationThreshold times (new conf parameter).

The recipients is also a conf value, FAILED_QUERIES_NOTIFICATION_RECIPIENTS.
A comma seperated email list is expected.
If it is empty no notifications are sent on db connection failure.

Change-Id: Id0d36b6140846bf98a33064cc69a9a398a8d4128
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=971324
Signed-off-by: Mooli Tayer <mtayer@redhat.com>