Accept multiple control SHAs from content service

[ktbartholomew]

GET https://content/control currently returns this:

{
  "sha": "6866e39c7725f56446de62e42ef087074d9bb123"
}

This works fine when a single control repo is involved and needs to be tracked, but falls short when rackerlabs/nexus-control#271 is finished and we have multiple repos to track to detect changes. In anticipation of this, the presenter needs to be able to parse the above payload, but also a payload like this:

{
  "sha": {
    "nexus-control": "abcdef",
    "developer-deconst-control": "abcdef",
    "carina-deconst-control": "abcdef"
  }
}

This will allow the presenter to update its control repos if any of them have changed, and potentially opens the door to a less obtrusive update process (e.g: only update the site that changed).