Bug 1863: LTS Support

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

Fixes bug 1863

Change-Id: If82a6f3169f7e800c6284c7f3e7f232b7fc73ea5
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.in b/requirements.in
index 6307ee1..3b86ffc 100644
--- a/requirements.in
+++ b/requirements.in
@@ -12,9 +12,10 @@
 # under the License.
 ##
 
-aiohttp
+aiohttp<3.8
+async-timeout<4
 grpcio-tools
 grpclib
-idna==2.10
+idna
 jinja2
-pyyaml
+pyyaml==5.4.1