Bug 1844: 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 1844

Change-Id: I66a80d649fa5beaaf0821c9e307ca7215ede9fd4
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.in b/requirements.in
index 8098683..a961d05 100644
--- a/requirements.in
+++ b/requirements.in
@@ -14,6 +14,6 @@
 CherryPy>=18.1.2
 jsonschema>=3.2.0
 python-keystoneclient
-pyyaml
+pyyaml==5.4.1
 requests
 tacacs_plus