8716 - K8s Proxy Charms

This commit enables the possibility to deploy
Proxy Charms on Kubernetes.

It also adds the k8s_cloud variable which will
state the name of the k8s that VCA will use to
deploy them.

Change-Id: I78763a67bdd51db60f7883fe01e04ac21170dfd6
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>