Do not assume order of device_ids set elements

This fixes the test_ancillary_bridges_multiple unit test that breaks with a randomized
PYTHONHASHSEED (see the bug report).
The test assumed that the device_ids set had
elements in a particular order. Found with PYTHONHASHSEED=2455351445.

The fix refactors the pullup_side_effect function so that it checks if the
device_id exists before returning the bridge.

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: Ie6b44d1d820ebb186036ffcd04ea2104dd384e94
Remove unnecessary _make_port function in BSN UTs

The Big Switch unit tests had unnecessary copies of the
_make_port function to allow the binding:host_id field to
be set. This was already possible with the existing _make_port
call through the use of kwargs so the extra function wasn't
necessary.

Change-Id: I88679173117739f628ae289636b5cdea10417e6c
Merge "Do not assume order of device_ids set elements"
Merge "Remove unnecessary _make_port function in BSN UTs"