Fix race in Postgres user-list

[Petr Malik]

Certain postgresql operations take quite some time.
The current implementation of user-list suffers
from a race condition (manifested on the gate).

The list is constructed by first retrieving
all user names from the database and then querying
again for access rights of each of them.

Apart from being inefficient (need to query for every user)
if a change (say, rename) occurs in between, the code subsequently
attempts to retrieve permissions on a non-existing username and
fails.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/361477

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/361477
Committed: https://git.openstack.org/cgit/openstack/trove/commit/?id=5c4b71000030c97c309167bc54445279e0224121
Submitter: Jenkins
Branch: master

commit 5c4b71000030c97c309167bc54445279e0224121
Author: Petr Malik <email address hidden>
Date: Fri Aug 26 17:19:54 2016 -0400

    Fix race in Postgres user-list

    Retrieve the user names and access rights
    in a single query hence closing the race
    window.

    Change-Id: I66847827fa8cb0615c1662670017d2cc55466ff3
    Closes-Bug: 1617464