Remove oauth2client._helpers dependency

[helen-fornazier]

oauth2client is deprecated
Migrate required helper's code from oauth2client to googleapiclient to
decrease our dependency on oauth2client

_helpers.py copied from oauth2client code
https://github.com/google/oauth2client/blob/master/oauth2client/_helpers.py

[helen-fornazier]

Please review, I am not entirely sure if this is the appropriated solution, please let me know what you think (I'll fix the errors in tox, I just would like some feedback if this is ok).
With this change plus #492 I can already manipulate instances using just google-auth, after uninstalling oauth2client

More specifically, what I am trying to run with google-auth instead of oauth2client is:
[code]: https://github.com/ClusterLabs/fence-agents/blob/master/fence/agents/gce/fence_gce.py
[PR]: ClusterLabs/fence-agents#197

Thanks

[theacodes]

This is actually great, @helen-fornazier! Could you also copy over the tests for those helpers from oauth2client?

[helen-fornazier]

@jonparrott done, I imported the tests too as suggested. Could you please take a look in the commits?

Thanks

[helen-fornazier]

Also, now the only file inside googleapiclient/ who uses oauth2client is googleapiclient/sample_tools.py, do you know if this module is used by external libraries?
It seems to me that this file is just an utility module used by samples/, so I was wondering if we could move sample_tools.pyto the samples/ folder and completely remove oauth2client from the install_requires list in the setup.py

[theacodes]

We can't remove sample_tools.py without causing a major-version breaking change, unfortunately. We can make it be helpful if oauth2client isn't available, something like this:

try:
    import oauth2cient
except:
    raise ImportError('googleapiclient.sample_tools requires oauth2client. Please install oauth2client and try again.')

[helen-fornazier]

@jonparrott lgtm, I added your suggestion in the PR, could you take a look please?

Can I remove oauth2client from the install_requires list or replace it by google-auth? https://github.com/google/google-api-python-client/blob/master/setup.py#L67

[theacodes]

LGTM with one tiny nit.

[theacodes]

Can I remove oauth2client from the install_requires list or replace it by google-auth?

Yes, but let's do it in two releases: one to have this, and then a second one to remove oauth2client.

[theacodes]

@helen-fornazier thank you so much for doing this!