New MockTests: TargetHttpProxyApi, UrlMap, ZoneApi, FirewallApi

I've seen this class still exists to test the `PATCH` method. Mock tests should also support it. In fact, as opposed to *expect* tests, mock tests perform *real* requests against a mocked backend, so we could say that if a Mock test doesn't work, the API does not work.

In order to bypass the limitation of the default JRE HttpUrlConnection implementation that does not support PATCH requests with a body, could you configure the tests to use the [OkHttp driver](https://github.com/jclouds/jclouds/tree/master/drivers/okhttp)? It should be as easy as just adding the `OkHttpCommandExecutorServiceModule` module to the set of modules in the `BaseGoogleComputeEngineApiMockTest` class. Could you give it a try? If it works we should also consider configuring the OkHttp driver as the default HTTP driver for GCE, to make sure all the methods work out of the box.