Update Dockerfile and stage-test script to run tests for charms

This patch updates Dockerfile to use Ubuntu 20.04 as base for building
and testing charms.
This patch updates stage-test script to execute testing for charms.
Tests will be executed only for charms modified by the review.
This patch updates tox configuration for charms setting the python
interpreter to python3.8.

Change-Id: Ib9046b78d6520188cc51ac776fe60ea16479f11c
Signed-off-by: Dario Faccin <dario.faccin@canonical.com>