Bug 1850: 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
Pin version of regexp to match IM

Fixes bug 1850

Change-Id: Ic9a4a2d32593f1f64f5426c733609787925fe88c
Signed-off-by: beierlm <mark.beierl@canonical.com>