mongdb get_event_types overly complex

[gordon chung]

currently, mongodb's implementation of get_event_types gets all events, loops, through said events and builds list of unique event_types.

this can be done in one line and optimised by db: db.event.distinct('event_type')

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/160522

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/160522
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=1e5f830fe76b96731d7a95f2baf099a3334bbe3a
Submitter: Jenkins
Branch: master

commit 1e5f830fe76b96731d7a95f2baf099a3334bbe3a
Author: gordon chung <email address hidden>
Date: Mon Mar 2 16:41:02 2015 -0500

    use mongodb distinct

    we should use distinct capability mongo provides and not try to do
    it ourselves.

    Change-Id: I8a77308475e3e0b12430fcff0496caa1905eef10
    Closes-Bug: #1427393