New kubernetes-client relation

[Ryan Farrell]

We are working to create a new charm for externally monitoring health of the kubernetes cluster in which the charm requires access to the kube-api. Currently we can use the two existing relations kube-api-endpoint, which provides the api url + port and kube-control, which contains a client token used to authenticate with the kube-api-server. This currently allows access to the API but since we are lacking the trusted ssl ca cert, it must be done with no cert checks, unless kubernetes-service-checks also relates to easyrsa with the certificates relation. This seems a bit heavy.

Instead of consuming 3 relatons to just act as a client to k8s, kubernetes-master charm ought to offer a new relation "kubernetes-client" which can provide all the necessary information through a single interface:

  k8s-api-hostname or ip,
  k8s-api-port
  trusted_ssl_ca
  client_token