Add support for SSL registry

[beeender]

• Add option 'ssl.ca.location', 'ssl.certificate.location',
  'ssl.key.location' and 'ssl.enabled.min_protocol' to support SSL
  enabled registry service.

Limitations:

• 'schema.registry.url' accepts multiple URLs separated by ','. But
  the options added by this commit don't. That means all the URLs will
  use the same SSL information provided by those options.

Close #7

[beeender]

BTW, I noticed this may not be the direction of #36 . It would be really good if the refactor work of #36 could handle the SSL connection.

[edenhill]

[clabot:check]

[ghost]

@confluentinc It looks like @beeender just signed our Contributor License Agreement. 👍

Always at your service,

clabot

[jarrodu]

Any word on this?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[alecbcs]

Hi all, is this PR still waiting on @beeender to sign the license agreement? Supporting SSL registries would be hugely helpful.

[beeender]

Hi all, is this PR still waiting on @beeender to sign the license agreement? Supporting SSL registries would be hugely helpful.

I did! 😄

[vdeters]

Looks good to me.