roles: Inherit from global vars with defaults

Global options such as 'timezone' or 'puid' can be set on a host and all
(relevant) roles will inherit them. Will be used for more variables in
the future.

roles/arr/defaults/main.yml

@@ -1,14 +1,16 @@
 ---
 
-arrstack_env_dir: /opt/arrstack
+# inherited from global
+arrstack_tz: "{{ timezone | default('America/Chicago') }}"
+arrstack_puid: "{{ puid | default(1000) }}"
+arrstack_pgid: "{{ pgid | default(100) }}"
+arrstack_umask_set: "{{ umask_set | default(022) }}"
 
+arrstack_env_dir: /opt/arrstack
 arrstack_serve_dir: /srv
 arrstack_serve_dir_create: true
 
-arrstack_puid: 1000
-arrstack_pgid: 100
-arrstack_tz: America/Chicago
-arrstack_umask_set: 022
-
 # arrstack_mb_user: Musicbrainz-user
 # arrstack_mb_pass: Musicbrainz-password
+
+# TODO: add commented version of remaining required vars (qbit, vpn)