Add API endpoint to query build information

This creates the '/build_info' endpoint and returns the build revision
as defined in the 'heat_revision' configuration option. This provides
both the revision of the engine and the api:

{
  "api": {
    "revision": <api revision>
  },
  "engine": {
    "revision": <engine revision>
  }
}

Implements: blueprint heat-build-info
Change-Id: Ibb2f5afda7c5d7c314e5e062d618405d9f4eb318