Upgrade to libjuju 2.9.2

- The upgrade to libjuju enables Pebble support
- Libjuju requires a newer version of Kubernetes so I updated it to the
latest one
- Additionally, a group of functions were moved from k8s_juju_conn.p to
kubectl.py, because they fit better there.

Related feature number: 10884

Change-Id: I354a2f49e7fc7b87a204bf60131a8d52a4f74cac
Signed-off-by: David Garcia <david.garcia@canonical.com>