Add MongoDB to Jenkins nodes

This is going to be needed by Ceilometer for unit testing.

Change-Id: Id353e7e3a829491bfb718f3c663054dc1430fcd6
Signed-off-by: Julien Danjou <julien@danjou.info>
Reviewed-on: https://review.openstack.org/33298
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins