Simplify molecule tests

"Upgrade" path includes multiple paths anyway:
- installing the latest official release package
- builing the current PR from source
- upgrading between the two versions

ansible/molecule/default/prepare.yml

@@ -0,0 +1,10 @@
+- name: install previous release
+  hosts: all
+  tasks:
+    - name: set previous version as installation target
+      set_fact:
+        paperlessng_version: latest
+
+    - name: install previous paperless-ng release
+      include_role:
+        name: ansible