Add pass-through config option for setting extra_sans

[lazypower]

As illustrated by this paste from a conversation: the primary consumer of this layer is kubernetes. And it appears to me that there should also be pass-through config options for extra_sans as users will inevitably bind a domain to an endpoint that is secured with these certificates.

http://paste.ubuntu.com/23996697/

[hyperbolic2346]

This config option was added to the kubernetes-master charm in PR 54234 and I'm working on getting the PR for the load balancer out now.