Bug 1848: 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 1848

Change-Id: I203d2d3f9ba636dfc12e0a640bdf445529713402
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements-test.txt b/requirements-test.txt
index d0c984f..7589d84 100644
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -1,13 +1,3 @@
-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.
 #
@@ -24,3 +14,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+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