Bug 1847: LTS Support

Updates base to Ubuntu 20.04
Updates python to 3.8
Updates pip dependencies to more recent versions
Adds apt cache to stage 2 dockerfile

Fixes bug 1847

Change-Id: Icbe9e9efc0edd957a4b8c0c461cf9d32c716df3e
Signed-off-by: beierlm <mark.beierl@canonical.com>