Add support for explicit cacert and client cert

[glance-]

This adds flags to registry-cli to be able to specify ca certificate and client certificates to use when validating and talking to a registry service.

[ipavlushin]

@glance- thanks for your contribution. Could you add tests?

[glance-]

I'm not working with the project, and at the company this code was written for, so I don't have any work-time to spend on this.

Either just merge as is, classify it as abandoned or maybe in another year or two I might find this useful again and pick it up.

[glance-]

Also, its impossible to write any meaningful unit-tests for a feature like this. It would require one to set up quite extensive integration tests with tls proxies, CA's and such.

So, no, I won't be writing any tests for this.