Do not assume order of body and tags elements

This fixes the l2gateway unit test that breaks with a randomized PYTHONHASHSEED
(see the bug report).

The test assumed that the body dict from self._create_expected_req_body
had elements (including contents of tags list) in a particular order.
Found with PYTHONHASHSEED=2455351445.
The fix ensures that body is in predictable order.

Partial-bug: #1348818
Note: There are several other unrelated unit tests that also break with a
randomized PYTHONHASHSEED, but they are not addressed here. They will be
addressed in separate patches.

Change-Id: I423b68aff58486c113d0e5c5f4726f9eabf6920e
Merge "Do not assume order of body and tags elements"
Merge branch 'master' of https://github.com/openstack/quantum