Support for GOOGLE_CLOUD_KEYFILE_JSON as in ruby client library

[phoet]

The ruby clients support passing the client configuration as an environment variable (GOOGLE_CLOUD_KEYFILE_JSON) cointaining a json blob instead of a path to a json config file.

https://github.com/googleapis/google-cloud-ruby/blob/1a7cfef70298276a9203658748e9d70edfcfb2d5/google-cloud-datastore/samples/README.md#authenticating

It would be super helpful (and less confusing) if there was a similar option in the python library.

[tseaver]

Related: googleapis/python-cloud-core#54 adds a from_service_account_info factory to the client base class, which would allow for the following:

import os
from google.cloud.storage import Client

keyfile_json = os.getenv("GOOGLE_CLOUD_KEYFILE_JSON")
if keyfile_json is not None:
    client = Client.from_service_account_info(keyfile_json)
else:  # fall back to normal env vars
    client = Client()

[tseaver]

Closing, given that the api-core PR has landed, and the spelling above accomplishes the requested usage.