[beta.2] service-account should be generated for kubectl gcp plugins

[abelanger5]

The Porter server needs to connect with an arbitrary number of GKE clusters using service account credentials. This sort of arbitrary service account switching is difficult -- there was a PR here that attempted to add this functionality using service account keys. As discussed in the issue, it doesn't make sense to implement this functionality using kubeconfig-based auth, and also doesn't make sense to download service account keys for each cluster and link the key files (for one, this would lead to a bunch of key files written in the container, which seems unsafe).

We'll need support for more idiomatic ways of connecting to clusters, and we'll likely have to drop the []byte storage of the kubeconfig in favor of non-kubeconfig based auth. The solution here is based on the following sources: [1], [2], [3]:

1. Attempt to infer the GCP project_id automatically. Query the user if the project_id is correct -- if it is not, or it is not possible to find a project_id, ask the user to input a project_id.

2. Create a service-account in that project using the iam admin package -- equivalent gcloud command:

gcloud iam service-accounts create porter-dashboard

3. Add policy binding to the service account using the iam package (will have to configure the roles differently depending on a provisioner/connector type) -- equivalent gcloud command:

gcloud projects add-iam-policy-binding PROJECT_ID \
    --member=serviceAccount:porter-dashboard@PROJECT_ID.iam.gserviceaccount.com
    --role=roles/container.developer

4. Get the service account credentials and store them in the DB.

5. Do something like the following to generate the client config:

func ClientFromSAKeyFile(ctx context.Context, filename string, scopes ...string) *rest.RESTClient, error {
	b, err := ioutil.ReadFile(filename)
	if err != nil {
		return nil, err
	}
	creds, err := google.CredentialsFromJSON(ctx, b, scopes...)
	if err != nil {
		return nil, err
	}
	rest.RESTClientFor(&rest.Config{
		Transport: &oauth2.Transport{
			Source: creds.TokenSource,
		}
	}), nil
}