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-test.txt b/requirements-test.txt
index 3b87659..7507a54 100644
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -1,15 +1,3 @@
-asynctest==0.13.0
-    # via -r requirements-test.in
-coverage==5.5
-    # via
-    #   -r requirements-test.in
-    #   nose2
-mock==4.0.3
-    # via -r requirements-test.in
-nose2==0.10.0
-    # via -r requirements-test.in
-six==1.15.0
-    # via nose2
 #######################################################################################
 # Copyright ETSI Contributors and Others.
 #
@@ -26,3 +14,15 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+asynctest==0.13.0
+    # via -r requirements-test.in
+coverage==6.2
+    # via
+    #   -r requirements-test.in
+    #   nose2
+mock==4.0.3
+    # via -r requirements-test.in
+nose2==0.10.0
+    # via -r requirements-test.in
+six==1.16.0
+    # via nose2