expose service git-revision in json config

We need to be able to specify service versions in the config, for repeatable deployments.

The config might look like this:

```
{
  "nova": {
    "git-rev": "bf770bd02b9376e9b0dbcdff9e1d71f721aee898",
      - or -
    "git-rev": "stable/folsom"
  ...
```
