Replace oauth2client with google-auth and oauthlib

[farmdawgnation]

oauth2client is deprecated and doesn't behave well when credentials are presented of the authorized_user type or provide built-in support for application default credentials. Updating the new libraries such that we could use the authorized_user credentials would avoid us having to populate a secret to our local dev environments.

[qiwzhang]

#706

[qiwzhang]

#706 is causing some problems. Hence this issue is revisited to see if the changes actually address your problems.

oauth2client is used to generate access_token from a service account json file to access Google services. There are two parts of code generating access token:

1. python code in start_esp.py. It is using oauth2client. This is for ESP to talk to Google service management service to fetch service config before Nginx starts.
2. c++ code inside Nginx. This is for actual requests to call Google service control service.

Even we change python code to replace oauth2client, but we could not change c++ code for that. Not sure if it will address your local dev issues by replacing oauth2client.

[farmdawgnation]

Last I checked, the python code was explicitly checking the type of the credentials presented were service_account so my authorized_user credentials didn't cut it.

Let me see if I can find time to test this out in the next few days and get back to you. (I was in a training class all of last week)

[qiwzhang]

Well, we fixed the problem of using google-auth. It is a forward moving step to replace deprecated oauthlib.