Fix changes-since filter for list-servers API

On list-servers API, "changes-since" parameter filters out servers
which have been created at the same date as the specified timestamp.
For example, the following server is filtered out with "changes-since=
2013-12-05T15:03:25":

  mysql> select display_name, updated_at from instances;
  +--------------+---------------------+
  | display_name | updated_at          |
  +--------------+---------------------+
  | vm01         | 2013-12-05 15:03:25 |
  +--------------+---------------------+

This bug causes some Tempest test failures, if the timestamp of server
creations are the same as the "changes-since" timestamp in second unit.
In addition, the behavior would be wrong from the viewpoint of its name
which includes "since".

This patch chanegs the server list for including the ones created at
the specified timestamp.

Change-Id: Icc10a1363503b3553d810c6be20b86b3da7ac1a0
Closes-Bug: #1236585
Merge "Fix changes-since filter for list-servers API"