datastore: use new DATASTORE_PROJECT_ID environment variable
datastore: use new DATASTORE_PROJECT_ID environment variable

Also, only warn about legacy environment variables if the new ones
aren't being used. env-init outputs both the new and old ones.

See https://cloud.google.com/datastore/docs/tools/datastore-emulator

Change-Id: I59648cd2fdad05e24b9cd74f4aba01aea982348b
Reviewed-on: https://code-review.googlesource.com/4460
Reviewed-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

googleapis/google-cloud-go,michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go