Support GOOGLE_BACKEND_CREDENTIALS env var

[Voziv]

I may be mistaken, but it seems this plugin does not allow the usage of GOOGLE_CREDENTIALS / GOOGLE_BACKEND_CREDENTIALS which is very confusing when the gcs remote state supports loading the service account key from these env vars.

I ran into this issue when trying to migrate our terraform CI to use terraspace instead as we have both of the above env vars defined.

GOOGLE_BACKEND_CREDENTIALS should be used first as the bucket could be in a different project from the GCP resources being deployed.

I'm seeing this with terraspace version 0.6.13 & ruby 2.7.0

[Voziv]

Looking a bit deeper I think my misunderstanding is that terraspace is using the google storage plugin for ruby which is different than the terraform gcs backend plugin.

The GCS plugin uses the environment variables as I've described. However the google storage plugin accepts it's own set of environment variables.

https://github.com/googleapis/google-cloud-ruby/blob/719c421b316cf9eb21df3b47abed73a6b0444c84/google-cloud-storage/AUTHENTICATION.md#environment-variables

We haven't used these to craft a solution yet. I will try to make sure we share whatever we do come up with.

If I had a vote though I'd say the best option for terraspace would be to have this align with terraforms GOOGLE_BACKEND_CREDENTIALS to keep things all in-line.