Removing hard dependency on PyOpenSSL.

[dhermes]

Requires oauth2client>=1.4.6 (@tseaver should I put this in setup.py?)

Also had to update storage/acl.py due to a change in
PyLint that didn't like self as a kwarg to a str instance
method

Fixes #537

[tseaver]

Given that we import oauth2client.crypt unconditionally, yes we need to bound it in install_requires.

BTW, 1.4.6 seems like an odd version number for a new feature (rather than a bug fix we need).

[coveralls]

Coverage remained the same when pulling a17cafe on dhermes:fix-537 into edfd5e2 on GoogleCloudPlatform:master.

[dhermes]

@tseaver It's a bit funky how OpenSSL is a "dependency" of oauth2client. The method will always be defined, but if OpenSSL is not installed then the method is defined as:

  def pkcs12_key_as_pem(*args, **kwargs):
    raise NotImplementedError('pkcs12_key_as_pem requires OpenSSL.')

For our library, the only place we need OpenSSL is the line in _get_pem_key in this PR. So if we decided not to support .p12 keys, we could totally ditch this. (It's unclear what Google's preference is since it's still possible to download a JSON/PEM key and a .p12 key.)

RE: The odd release number, this was just added in googleapis/oauth2client#115

[tseaver]

@craigcitro @dhermes the setuptools way to express "optional" dependencies is via extras. I don't know if that would clarify anything for oauth2client.

[dhermes]

I am a n00b there and happy to learn. This extra will be for oauth2client, not for us, so may not be blocking for this PR. Am I missing something?

[tseaver]

Nothing blocking us now. If oauth2client releases a version that has an [openssl] extra, we would need to adjust our dependency to pick that up.

LGTM to merge.