Sort _get_new/deleted_set_ips responses in unittests

Sort _get_new/deleted_set_ips responses in unittests

This fixes the test_set_members_adding/deleting_less_than_5 unit test
that breaks with a randomized PYTHONHASHSEED (see the bug report).

The test assumed that the _get_new/deleted_set_ips from
neutron.agent.linux.ipset_manager return elements in a particular order.
Found with PYTHONHASHSEED=1.

The fix refactors the test case to force sorted responses from
_get_new/deleted_set_ips during unittests.

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: I8408365825ec1e97a83c2181f38ec1f9468df91e