allow setting the service account email when creating a google batch CE

[JaimeSeqLabs]

Description

Closes #316

Allow setting the service account email address as configuration field when creating a new Google Batch compute environment.

Guidelines for testing

Create a new Google Batch CE setting the service account email address:

$> ./tw compute-envs add google-batch -n GoogleBatchTest -c JaimeGoogleCredsDev --work-dir=gs://jaime-test-workdir --location=europe-west2 --service-account-email=<YOUR_SERVICE_ACCOUNT_EMAIL>