Update director-configure-blobstore for GCS
Update director-configure-blobstore for GCS

BOSH director and bosh-cli have both merged support for using
GCS as a director blobstore. This change documents GCS usage.
https://github.com/cloudfoundry/bosh/pull/1732
https://github.com/cloudfoundry/bosh-cli/pull/238

cloudfoundry/docs-bosh