Build an image for heat functional tests

This is only triggered if HEAT_CREATE_TEST_IMAGE is True

This custom image contains the following:
* heat-cfntools from git rather than the packaged version, which
  will allow gating on heat-cfntools changes
* os-collect-config, os-apply-config, os-refresh-config, which
  will allow gating on these projects, and will allow heat
  software-config tests to be written
* software-config hooks from heat-templates, which will allow tempest
  tests to be written for software-config with different configuration
  tools (script, puppet, cfn-init etc)

The heat functional tests will soon replace the heat-slow tempest job,
so heat-slow tempest configuration will be removed after the heat
functional test job is gating.

Change-Id: I2e0490c1662a184d4c6d8c7e9ebb128e1912f1b0
Merge "Build an image for heat functional tests"