Unit test of 55-heat-config

This implements a unit test of the os-refresh-config script 55-heat-config.

The test does the following:
- hook-fake.py is a fake hook which is executed for every config item
  found by 55-heat-config
- test_heat_config.HeatConfigTest does the following:
  - creates a temp dir and copies hook-fake.py into it for every
    hook/group name the test expects (puppet, script etc)
  - writes our sample config json and invokes 55-heat-config
    to trigger the execution of the fake hook for each config item
  - assert that the fake hooks wrote out the expected json for each
    config item

Change-Id: I87c108dc07dacde767b7f74c82c7ae06ab765738