roles: Make domain global variable

Each role has their own subdomains which can be set manually, or can
inherit from the global domain with a default prefix.

roles/paperless/defaults/main.yml

@@ -1,6 +1,7 @@
 ---
 
 # inherited from global
+stack_paperless_subdomain: "documents.{{ domain | default('example.org') }}"
 stack_paperless_tz: "{{ timezone | default('America/Chicago') }}"
 stack_paperless_puid: "{{ puid | default(1000) }}"
 stack_paperless_pgid: "{{ pgid | default(100) }}"