Feature 10892/10893/8460: refactor of OSM installer

This change covers:

- Feature 10892. Installation of OSM on top of Ubuntu20.04. Changes
  are mostly in full_install_osm.sh and are related to the use of new
  versions of kubeadm and docker-ce. In addition, changes in Jenkins
  groovy files have been done to indicate the base image to be used,
  either 18.04 or 20.04.
- Feature 10893. Better tracking of installation. The code for tracking
  in in common/track. There is a function track that it is called in
  the different steps of the installation.
- Feature 8460: Cleanup old code in full_install_osm.sh. The script
  full_install_osm.sh has been split in different scripts performing
  specific tasks, thus simplifying the installer: install_docker_ce.sh,
  install_juju.sh and install_kubeadm_cluster.sh.

Change-Id: I1e388ec56285337eaf34f68470aa5a9b23ff45ff
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>