datastore: don't require auth with local datastore server

[stephenplusplus]

Fixes #936

When a service object is created (such as a Dataset object), we have the choice of telling our "makeAuthenticatedRequestFactory" function that we don't care about the authentication part via an option called customEndpoint. A Dataset is the one place we use this option in the event a user is trying to communicate with their own Datastore server.

We were skipping the auth when:

• A user instantiated a Dataset providing an apiEndpoint option:

  var dataset = datastore.dataset({ apiEndpoint: 'http://localhost:8080' });

But, we also need to support when:

• A user has the DATASTORE_HOST env var set

We have had half of the puzzle solved; we detect the presence of that env var and send the API requests to it accordingly. But, we forgot to update customEndpoint, the option that skips authenticating.

// @brainsiq

[jgeewax]

LGTM

[callmehiphop]

👍