Use "password" for additional Postgres databases, not "pass"

Being more explicit sounds better.

roles/matrix-postgres/tasks/util/create_additional_database.yml

@@ -6,7 +6,7 @@
   fail:
     msg: "Additional database definition ({{ additional_db }} lacks a required key: {{ item }}"
   when: "item not in additional_db"
-  with_items: "{{ ['name', 'username', 'pass'] }}"
+  with_items: "{{ ['name', 'username', 'password'] }}"
 
 # The SQL statements that we'll run against Postgres are stored in a file that others can't read.
 # This file will be mounted into the container and fed to Postgres.