Display current heat build information

It is useful for system administrators, support users, and possibly regular users to be able to query current build information. An authenticated user could query /v1/build_info and get information about the current heat build that would be retrieved from a configuration file (e.g. etc/heat/heat.conf). The response could look like this:

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

The full spec contains more details.