OSM LTS Support

Cherry-picks:
  - 53dd746f - bugfix: kubeconfig passed for helm commands. Bug 1789
  - 7114f65c - Fix bug 1722: add support for deploying .charm files
  - 42e51cfd - Fix bug 1715
  - 856d33b9 - Update requirements.txt and tox.ini
  - a39cca06 - Bug 1651 fix
  - 76ed7579 - Allow the deployment of privileged KDUs
  - ba1d0726 - Remove unsecure and unused function
  - e610aedb - Fix bug 1589: Improve deletion of models
  - 4ae527ee - Unit tests improvements
  - d8d4b6e7 - Upgrade to libjuju 2.9.4

Other:
  - Use Ubuntu 20.04 (focal) as default series for Proxy Charms

Change-Id: Ie7bc236723f7ce96887e48e266a94551dd661358
Signed-off-by: David Garcia <david.garcia@canonical.com>