Add Posibility to pass the gcp sevice account credentials without having to define an environment variable all the time

[RubenBBlazquez]

Add the posibility to pass the service account credentials into the GcpCluster and after that in the GCP Compute to authenticate without having to define an environment variable all the time, because if you work with workers and each worker is independent, set an environment variable all the time is not the best approach

[dbalabka]

@RubenBBlazquez, I've created a Dask plugin that automatically uploads the GCP JSON credentials file using the path specified in GOOGLE_APPLICATION_CREDENTIALS. In this case, you don't need to specify the ENV manually, it happens automatically. It works great for the development enviroment when we want all cloud infrastructure manipulations and API calls authorized by developer's personal account. Please see PR: #439
Would it work for you? Is this your case?

Reference: https://cloud.google.com/iam/docs/best-practices-service-accounts