SASL SCRAM support

[wsmith-twc]

It looks like someone beat me to a pull request by a few hours, but I thought I'd submit this in case this code is useful.

I've used against Kafka 2.2.1 and 2.3.0 with Python 2.7.15 and 3.6.8. I've only used security mechanism of SCRAM-SHA-512 (not SCRAM-SHA-256). I've tried it with security protocols of SASL_PLAINTEXT and SASL_SSL.

The code re-uses sasl_plain_username and sasl_plain_password. That makes these names a little inaccurate, but having separate sasl_scram_username and sasl_scram_password didn’t seem like a great option, either. Another option would be to introduce sasl_username and sasl_password that both mechanisms use and deprecate sasl_plain_*

I didn’t follow the exact style of conn._try_authentiate_plain and conn._try_authenticate_gssapi because there are more errors that can occur. I relied on exceptions more than in those functions.

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This change is 

[dpkp]

Thanks!! I'm going to go with #1918 primarily because it includes sasl fixture support + tests, which also benefit existing sasl plain mechanisms. I agree with your thinking on sasl_plain_* configs. Thanks again for working on this.