TLS Termination Support

[chuckbutler]

• Adds methods to set/get the client certificates
• Renamed the provide_client_credentials method to set_client_credentails (retained old method for backwords compat)
• updates readme with instructions
• Adds method(s) to set bits on disk when interacting with the client certificates

[AdamIsrael]

From a perfunctory review, I'd say the code is fine. Is there a way to add unit tests to an interface, or would we need a charm/layer to exercise this functionality?

[chuckbutler]

I think in this particular case we'll need a charm, and will need bundle tests. This is a prelim submission for Tigra (formerly Calico) - who is the primary stakeholder behind the etcd-proxy interface currently.

I'm only aware of their charm using it, and it carries with it an openstack + neutron dependency. A bit heavy handed for a test. We can probably shape something up similar to validate data sent on the wire with a dummy charm.