Implicit set_ methods should return the values set and raise if not successful

[dhermes]

See @tseaver comment about returning (and the current storage regression test hacky behavior).

This will likely require doing something like

class Environment(object):
    ....

DEFAULT_ENVIRON = Environment()

def set_default_foo():
    # Do some stuff to DEFAULT_ENVIRON
    return DEFAULT_ENVIRON

[dhermes]

@tseaver We currently have 7 set_ methods defined:

$ git grep -n 'def set_'
gcloud/datastore/__init__.py:65:def set_defaults(dataset_id=None, connection=None):
gcloud/datastore/_implicit_environ.py:131:def set_default_dataset_id(dataset_id=None):
gcloud/datastore/_implicit_environ.py:185:def set_default_connection(connection=None):
gcloud/storage/__init__.py:56:def set_default_bucket(bucket=None):
gcloud/storage/__init__.py:82:def set_default_project(project=None):
gcloud/storage/__init__.py:101:def set_default_connection(project=None, connection=None):
gcloud/storage/__init__.py:117:def set_defaults(bucket=None, project=None, connection=None):
$ git log -1 --pretty=%H
3c7316ee312a3255fd78aa93d136ec5b19060417

Among what we do have:

• datastore._implicit_environ.set_default_dataset_id throws an exception if not set but does not return
• datastore._implicit_environ.set_default_connection neither throws nor returns
• datastore.__init__.set_defaults just wraps set_default_dataset_id and set_default_connection, so could get exception throwing for free but returning is not simple. We could just remove this since we have the lazy loading now.
• storage.__init__.set_default_bucket neither returns nor throws
• storage.__init__.set_default_project neither returns nor throws
• storage.__init__.set_default_connection neither returns nor throws
• storage.__init__.set_default_defaults neither returns nor throws

[dhermes]

Irrelevant in Client world