Add Ubuntu 20.04 to CI (#1658)

* Add Ubuntu 20.04 to CI.

* Skip mqtt test on Ubuntu 20.04.

* Use explicit disable instead of aliases.

* Add Ubuntu 20.04 specific config for setup_postgresql_db.

* Fix postgres version.

tests/integration/targets/mqtt/tasks/main.yml

@@ -6,4 +6,4 @@
 - include: ubuntu.yml
   when: 
     - ansible_distribution == 'Ubuntu'
-    - ansible_distribution_release != 'trusty'
+    - ansible_distribution_release not in ['trusty', 'focal']

tests/integration/targets/setup_postgresql_db/vars/Ubuntu-20-py3.yml

@@ -0,0 +1,8 @@
+postgresql_packages:
+  - "postgresql"
+  - "postgresql-common"
+  - "python3-psycopg2"
+
+pg_hba_location: "/etc/postgresql/12/main/pg_hba.conf"
+pg_dir: "/var/lib/postgresql/12/main"
+pg_ver: 12