Support Datastore default dataset env var

[stephenplusplus]

See https://github.com/GoogleCloudPlatform/gcloud-common/issues/24

We currently support DATASTORE_HOST, but not DATASTORE_DATASET.

[stephenplusplus]

@pcostell will DATASTORE_DATASET still be around in v1beta3? Are there other env vars we should look out for?

[pcostell]

DATASTORE_DATASET will go away in favor of DATASTORE_PROJECT_ID.

DATASTORE_HOST will go away and become DATASTORE_EMULATOR_HOST. However, it won't include the url scheme.

[stephenplusplus]

Thanks!