Add support for gcd local development server

Right now, there doesn't appear to be any support for the datastore local development server gcd. For instance, the usage instructions tell you to set the dataset id in the environment variable DATASTORE_DATASET but gcloud-python doesn't appear to look for that in the set_default_dataset_id() function.

It's not clear to me how exactly this is meant to work. I have set the environment variable DATASTORE_HOST to be the local development server as per the gcd instructions but it seems to be completely ignored by gcloud-python. It works with the googledatastore library so it would be nice if it worked here too.

@rstuart85

@rstuart85 Thanks for the request. I have not used gcd and will look into this. It should be easy enough to support in gcloud/datastore/connection.py (at 61c6814)

class Connection(connection.Connection):
    ...
    API_BASE_URL = os.getenv('DATASTORE_HOST', connection.Connection.API_BASE_URL)
    ...

and in gcloud/datastore/__init__.py

def set_default_dataset_id(dataset_id=None):
    if dataset_id is None:
        dataset_id = os.getenv(_DATASET_ENV_VAR_NAME)

    if dataset_id is None:
        dataset_id = os.getenv('DATASTORE_DATASET')

    if dataset_id is None:
        dataset_id = _implicit_environ.app_engine_id()

   ...

(check comes before App Engine but after the production environment variable)

Certainly seems straightforward and would really simplify local development of products that use Datastore. Hopefully it will also reduce latency when developing!

Adding support for GCD dev server in datastore package.