molecule scenario to test upgrading from releases

Provide a separate scenario to install the last release, then update to
the current one.
Also includes improved deployment verification (check if uploading a
.txt through the API works).

ansible/molecule/update/prepare.yml

@@ -0,0 +1,10 @@
+- name: install previous release
+  hosts: all
+  tasks:
+    - name: set previous version as installation target
+      set_fact:
+        paperlessng_version: 0.9.13
+
+    - name: install previous paperless-ng release
+      include_role:
+        name: ansible