Bug 1844: 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 1844

Change-Id: I66a80d649fa5beaaf0821c9e307ca7215ede9fd4
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements-dev.txt b/requirements-dev.txt
index d80246b..e02eb10 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,37 +1,3 @@
-aiokafka==0.7.0
-    # via osm-common
-bitarray==1.8.1
-    # via pyangbind
-dataclasses==0.6
-    # via osm-common
-enum34==1.1.10
-    # via pyangbind
-kafka-python==2.0.2
-    # via aiokafka
-lxml==4.6.3
-    # via
-    #   pyang
-    #   pyangbind
-git+https://osm.etsi.org/gerrit/osm/common.git@v10.0#egg=osm-common
-    # via -r requirements-dev.in
-git+https://osm.etsi.org/gerrit/osm/IM.git@v10.0#egg=osm-im
-    # via -r requirements-dev.in
-pyang==2.4.0
-    # via
-    #   osm-im
-    #   pyangbind
-pyangbind==0.8.1
-    # via osm-im
-pycrypto==2.6.1
-    # via osm-common
-pymongo==3.11.3
-    # via osm-common
-pyyaml==5.4.1
-    # via osm-common
-regex==2021.3.17
-    # via pyangbind
-six==1.15.0
-    # via pyangbind
 # Copyright 2018 Telefonica S.A.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -48,3 +14,50 @@
 # limitations under the License.
 
 
+aiokafka==0.7.2
+    # via -r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=v10.0
+bitarray==1.8.1
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   pyangbind
+dataclasses==0.6
+    # via -r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=v10.0
+enum34==1.1.10
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   pyangbind
+kafka-python==2.0.2
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   aiokafka
+lxml==4.6.3
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   pyang
+    #   pyangbind
+osm-common @ git+https://osm.etsi.org/gerrit/osm/common.git@master
+    # via -r requirements-dev.in
+osm-im @ git+https://osm.etsi.org/gerrit/osm/IM.git@master
+    # via -r requirements-dev.in
+pyang==2.4.0
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   pyangbind
+pyangbind==0.8.1
+    # via -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+pycrypto==2.6.1
+    # via -r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=v10.0
+pymongo==3.12.3
+    # via -r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=v10.0
+pyyaml==5.4.1
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   -r https://osm.etsi.org/gitweb/?p=osm/common.git;a=blob_plain;f=requirements.txt;hb=v10.0
+regex==2021.3.17
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   pyangbind
+six==1.15.0
+    # via
+    #   -r https://osm.etsi.org/gitweb/?p=osm/IM.git;a=blob_plain;f=requirements.txt;hb=v10.0
+    #   pyangbind