Add GAE/GCE support for datastore.set_default_dataset and add exception on failure

[dhermes]

See discussion in #444 for comments on exception.

Something like oauth2client.client._get_environment could be good to detect GAE/GCE dataset ID's but _detect_gce_environment should be better as discussed in googleapis/oauth2client#93

Not blocking for M1.

[dhermes]

Relevant to GAE:

>>> import os
>>> from google.appengine.api import app_identity
>>>
>>> print repr(os.getenv('APPLICATION_ID'))
's~foo'
>>> print repr(app_identity.get_application_id())
'foo'

[dhermes]

Relevant to GCE:

>>> import urllib2
>>> BASE = 'http://metadata.google.internal/'
>>> PROJECT_PATH = 'computeMetadata/v1/project/project-id'
>>> URI = BASE + PROJECT_PATH
>>> request = urllib2.Request(URI, headers={'Metadata-Flavor': 'Google'})
>>> response = urllib2.urlopen(request)
>>> contents = response.read()
>>> response.close()
>>> contents
'foo'

No prefix there.

[dhermes]

@tseaver I just realized that #572 does not raise when setting fails.

We can add this in a sweeping way for both packages after #572 goes through? We'll have to determine the difference between "required" defaults (always raise) and "optional" defaults (sometimes raise) for the set_defaults functions.

[dhermes]

Re-opening until exception questions resolved.