Gate-supporting files for functional tests

To support running functional tests from the gate:
- directory functionaltests for hook scripts (and maybe test packages)
- tox.ini functional environment placeholde
- devstack post_test_hook, pre_test_hook scripts which are run
  by the gate job

Change-Id: Iead98b60149807e26a4888c3497876483ca803b7
Partial-Blueprint: functional-tests
Merge "Gate-supporting files for functional tests"