JCLOUDS-336. Move to GCE v1beta16 API.

I've tested #28 for 1.6.3 and it failed 2 tests:
```
Failed tests:   testNetworksAndFirewallDeletedWhenAllGroupNodesAreTerminated(org.jclouds.googlecomputeengine.compute.GoogleComputeEngineServiceExpectTest): expected [GET https://www.googleapis.com/compute/v1beta15/projects/myproject/global/networks/jclouds-test-delete HTTP/1.1(..)
  testCreateNodeWhenNetworkNorFirewallExistDoesNotExist(org.jclouds.googlecomputeengine.compute.GoogleComputeEngineServiceExpectTest): expected [GET https://www.googleapis.com/compute/v1beta15/projects/myproject/global/firewalls/jclouds-test-port-22 HTTP/1.1(..)
```
I've opened this new PR that fixes them for [JCLOUDS-226](https://issues.apache.org/jira/browse/JCLOUDS-336)

Hope this helps