datastore: derive GCP project ID from GOOGLE_CLOUD_PROJECT.
datastore: derive GCP project ID from GOOGLE_CLOUD_PROJECT.

The App Engine Go standard environment stopped setting the
"DATASTORE_PROJECT_ID" environment variable (golang/appengine#178).
This change adds a fall-back to the "GOOGLE_CLOUD_PROJECT" environment
variable, which is documented for the Go 1.12+ standard environment.

Change-Id: Ic946827dc7978b1d68102b8c37222631340f1fd5
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/53290
Reviewed-by: Chris Cotter <b879ae7d4b52f11ce9f60d9854075eb8eebaf7a9@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go