Adding support for GCD dev server in datastore package.

[dhermes]

Enabling

• set_default_dataset_id() support for DATASTORE_DATASET environment
  variable name
• gcloud.datastore.Connection.API_BASE_URL override for
  DATASTORE_HOST environment variable name

Fixes #650.

[dhermes]

@tseaver Note the super-hacky way to test the import time class constant definition. I'd love a better way if anything comes to mind (I thought about a module level constant, but this still happens at import time)

[dhermes]

I tested and confirmed this works with gcd

[coveralls]

Coverage remained the same at 100.0% when pulling d2aaab8 on dhermes:fix-650 into 8e68a20 on GoogleCloudPlatform:master.

[tseaver]

I'm not sure about doing stuff at import time. Could we add a class method to update the various constants, and then call it from within set_defaults(), IFF the environment variable is present?

[dhermes]

I like the idea but am worried about the interdependence between the modules.

It may be possible to just use a api_base_url() class method since the base class never uses API_BASE_URL and our only use in datastore code is

api_base=(base_url or cls.API_BASE_URL),

[tseaver]

We could change build_api_url() to an instance method (the only non-test caller already dispatches through the instance) and then have set_defaults() (or its minions) stamp the GCD version on the constructed connection instance. I don't think that introduces any new coupling.

[dhermes]

The issue is that people may make a Connection in another way than set_default_connection().

[tseaver]

Then they can set the attribute on that instance (or pass it to the constructor, if we think supporting it is important enough).

[dhermes]

@tseaver PTAL

I didn't need to interact with set_default_...() since DATASTORE_HOST is used for this while DATASTORE_DATASET is used for the actual string ID.

UPDATE: I removed API_BASE_URL from the base class (so people don't mix up instance vs. class) and then just added to the constructor.

[coveralls]

Coverage remained the same at 100.0% when pulling b9aaf10 on dhermes:fix-650 into 8e68a20 on GoogleCloudPlatform:master.

[dhermes]

@tseaver How do you like this approach for #573?

[tseaver]

LGTM. I like the instance attr / method approach.