Do not assume order of lvm.tun_ofports set elements

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

The test assumed that the lvm.tun_ofports set had
elements in a particular order. Found with PYTHONHASHSEED=2455351445.

The fix refactors the add_fdb_flow function to sort the set before turning into
comma separated string.

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: I86b453a93f3ba09212709caf462cf3bfc5b21ee9