Disable module via config

[davidwindell]

At the moment, there is no way to disable the module via config options so the module can't be completely disabled per environment.

This has led to fatal errors in production (we only use the module on our staging cluster).

Will submit a merge request if I get some time.

[davidwindell]

We've implemented a check like this for now:

https://github.com/outeredge/googlecloudstorage/commit/82f5e57b0d9449929ede83b91c293ad4741ee08a