Bug 1842: 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 1842

Change-Id: Iefa3c72fe200ecf4f0b4081db76d203ac38a50e7
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements-test.txt b/requirements-test.txt
index 871fdef..9c6fd21 100644
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -1,27 +1,3 @@
-certifi==2020.12.5
-    # via requests
-chardet==4.0.0
-    # via requests
-coverage==5.5
-    # via
-    #   -r requirements-test.in
-    #   nose2
-idna==2.10
-    # via requests
-mock==4.0.3
-    # via -r requirements-test.in
-nose2==0.10.0
-    # via -r requirements-test.in
-requests-mock==1.8.0
-    # via -r requirements-test.in
-requests==2.25.1
-    # via requests-mock
-six==1.15.0
-    # via
-    #   nose2
-    #   requests-mock
-urllib3==1.26.4
-    # via requests
 #######################################################################################
 # Copyright ETSI Contributors and Others.
 #
@@ -38,3 +14,27 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+certifi==2021.10.8
+    # via requests
+charset-normalizer==2.0.10
+    # via requests
+coverage==6.2
+    # via
+    #   -r requirements-test.in
+    #   nose2
+idna==3.3
+    # via requests
+mock==4.0.3
+    # via -r requirements-test.in
+nose2==0.10.0
+    # via -r requirements-test.in
+requests==2.27.1
+    # via requests-mock
+requests-mock==1.9.3
+    # via -r requirements-test.in
+six==1.16.0
+    # via
+    #   nose2
+    #   requests-mock
+urllib3==1.26.8
+    # via requests