Bug 1840: 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 1840

Change-Id: I2cf79d9e697ba1267e2a8d00a180412a2887e1bd
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.in b/requirements.in
index 1ce6a65..b8e0f2e 100644
--- a/requirements.in
+++ b/requirements.in
@@ -13,8 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-pymongo
+pymongo<4
 aiokafka
-pyyaml
+pyyaml==5.4.1
 pycrypto
 dataclasses
\ No newline at end of file