[google|dns] Add initial support for Google Cloud DNS.

- some common plumbing
- requests for creating, deleting and listing managed zones, with tests

One thing we should discuss is the test domain in tests/google/requests/dns/managed_zone_tests.rb
For the test to pass in non-mocked mode, the developer who runs the tests needs to confirm its ownership with Google. This implies that it needs to be configured for every developer individually.
Alternatively (and this is what probably should be the default) is to disable most (if not all) the Google DNS tests in non-mocked mode.
Here's how tests now look in real and mock mode:

```
  Fog::DNS[:google] | managed_zone requests (google)
    success
      Needs a verified domain # success
    failure
      #delete_managed_zone + raises Fog::Errors::NotFound
  
  1 pending, 1 succeeded in 2.974651229 seconds
```

```
  Fog::DNS[:google] | managed_zone requests (google)
    success
      #create_managed_zone + data matches schema
      #list_managed_zones
        schema + data matches schema
        test zone present + returns true
      #delete_managed_zone + returns nil
    failure
      #delete_managed_zone + raises Fog::Errors::NotFound
  5 succeeded in 0.881664572 seconds
```