core : Modify MLA to use engine session info

Modify MLA queries to use engine session table with
id instead of the user_flat_groups view

Drop user_flat_groups view and unused columns from
Users table.

Change-Id: I28f3853166fe635941e65ea2461f7ecda20f4a2a
Bug-Url: https://bugzilla.redhat.com/1092744
Signed-off-by: Ravi Nori <rnori@redhat.com>