core: adding missing default to a non-null column

This patch fixes an issue in event_subscriber table that was added a
notification_method column which does not allow nulls.

If the table has any records before the upgrade, this will fail since
the existing records will have NULL for the new added column.

Change-Id: Id3c7ce30c32b9b38f08ff6c8c0eb8bfa02cf5066
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1072867
Signed-off-by: Eli Mesika <emesika@redhat.com>