datastore: add support for the datastore emulators
datastore: add support for the datastore emulators

The gcd and `gcloud emulators` tools provide datastore emulators,
exposed via environment variables. Automatically configure the client
based on these environment variables: DATASTORE_HOST, DATASTORE_DATASET

Change-Id: I6752005ea23700de28f03514690db32febc60fb2
Reviewed-on: https://code-review.googlesource.com/3350
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

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