Not necessarily needed for M1, just points in the direction of running in environments like GAE and GCE.

I took a stab at it using environment variables in dhermes@0860e59
In #444 this is done for datasets.

@craigcitro I tried to parse how getting auth values from the environment works in GetCredentials and BaseApiClient.__init__ but it was unclear (for example) how _API_KEY is set.