Change naming of Charmed Installer Parameters

The naming for the following parameters in the
Charmed installer has been changed:

kubeconfig -> k8s
controller -> vca
lxd-cloud -> lxd
lxd-credentials -> lxd-cred

Change-Id: Id760cd332c4be76a3cf45a108d7448387a78a94a
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>