Figure out what our "usable auth" story is for gcloud-python

[jgeewax]

There is some discussion going on now about a separate library that would just handle authentication. The goal of this issue is:

• figure out a plan to moving the authentication code into a separate library we depend on (apitools? googleauthkit (working name)?)
• make this library exist and add it as a dependency (if not already done)
• remove our custom authentication code and replace it with calls into this outside library to get an authenticated transport

(Forgive me if this is a duplicate issue...)

[tseaver]

ISTM that (with the exception of signing URL endpoints, which uses crypto, but is not really authnz), we already delegate pretty much all our authnz to oauth2client.

[dhermes]

Echo what @tseaver says. We don't have any custom auth behavior and the auth is quite usable.

[dhermes]

@jgeewax Closing this out because it's been in oauth2client all along. Please re-open if you disagree.