Bug 1841: 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
Change stage 3 base image to Ubuntu 20.04 so we can use
python3.8

Fixes bug 1841

Change-Id: I442d0ecf09f033946bbbc3301fd8de54382a4195
Signed-off-by: beierlm <mark.beierl@canonical.com>