unnecessary calls to method distinct('_id')

[Tong Li]

In impl_mongodb.py, there are few places which makes call like this distinct('_id'), since _id field is already unique in all collections, there is no need to do that call to waste time. That call should be removed or reworked on. Specifically in get_users and get_projects method.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/38537

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/38537
Committed: http://github.com/openstack/ceilometer/commit/4514eb87675ed9cf46d9fd4397727059b7881a37
Submitter: Jenkins
Branch: master

commit 4514eb87675ed9cf46d9fd4397727059b7881a37
Author: Tong Li <email address hidden>
Date: Thu Jul 25 10:33:55 2013 -0400

    calling distinct on _id field against a collection is slow

    To optimize the code, distinct method should be avoided especially
    against _id field. Sort should also be done on the server side to
    take advantage of generator to avoid using up memory for large
    data set.

    Change-Id: Ib417f73e2238a5d9865a2a1a9990e891b0f7a24c
    fixes: bug #1204607
    fixes: bug #1204609