Google: allow for different libcloud provider to support upcoming DNS mo...

This small fix provides support for additional libcloud providers. The Google modules in ansible utilize libcloud and currently all existing modules use the GCE libcloud provider. However, the libcloud DNS module uses a separate DNS provider.

Before I can contribute the Google DNS ansible module (pull-request coming to `ansible-modules-extras` soon), this must first be merged.

```sh
$ make tests
...
----------------------------------------------------------------------
Ran 179 tests in 8.642s

OK
```