GCP: Add support for specifying the backups storage bucket.

[jimright]

Incorporating PR#134 and required fixes to work with cloudera.cloud.env.

Note - to test creation and use of the backup storage bucket we found that we needed to specify both the infra.gcp.storage.path.backups and env.gcp.storage.path.backups variables:

env:
  gcp:
    storage:
      path:
        backups: "jegc-custom-backups"

infra:
  gcp:
    project: "{{ gcp_project_id }}"
    storage:
      path:
        backups: "jegc-custom-backups"