Queryable VM metadata

This PR allows for a checked-out VM's metadata to be queryable via REST, like:

    $ curl --url vmpooler/vm/e8pk9g08fctgcdm

    {
      "ok": true,
      "e8pk9g08fctgcdm": {
        "template": "centos-6-x86_64",
        "lifetime": 12,
        "running": 0,
        "domain": "company.com"
      }

Other small ride-along changed in this PR:

* Disable querying of hostnames sitting in 'ready' queue (via 'GET') (there isn't a use-case for this query, and it brings along security concerns)
* Allow FQDN when modifying a host (via 'PUT')
If a hostname does not exist, this should return a 404 instead of a 200 (OK) with an error code within the result payload.