Bug 1843: 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 1843

Change-Id: Iec7c73cb99f5ed3f0e07666d8c8c8be8ca91c0ea
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements-test.txt b/requirements-test.txt
index 7e3a14d..c6718ee 100644
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -1,37 +1,3 @@
-asynctest==0.13.0
-    # via -r requirements-test.in
-certifi==2020.12.5
-    # via requests
-chardet==4.0.0
-    # via requests
-coverage==5.5
-    # via
-    #   -r requirements-test.in
-    #   nose2
-flake8==3.9.0
-    # via -r requirements-test.in
-idna==2.10
-    # via requests
-mccabe==0.6.1
-    # via flake8
-mock==4.0.3
-    # via -r requirements-test.in
-nose2==0.10.0
-    # via -r requirements-test.in
-pycodestyle==2.7.0
-    # via flake8
-pyflakes==2.3.1
-    # via flake8
-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.
 #
@@ -48,3 +14,37 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+asynctest==0.13.0
+    # via -r requirements-test.in
+certifi==2021.10.8
+    # via requests
+charset-normalizer==2.0.10
+    # via requests
+coverage==6.2
+    # via
+    #   -r requirements-test.in
+    #   nose2
+flake8==4.0.1
+    # via -r requirements-test.in
+idna==3.3
+    # via requests
+mccabe==0.6.1
+    # via flake8
+mock==4.0.3
+    # via -r requirements-test.in
+nose2==0.10.0
+    # via -r requirements-test.in
+pycodestyle==2.8.0
+    # via flake8
+pyflakes==2.4.0
+    # via flake8
+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