Add prosody/xmpp default vars to role

group_vars/sovereign

 # irc_quitmsg: (required)
 # irc_password_hash: (required)
 # irc_password_salt: (required)
+
+# xmpp
+# ----
+# prosody_admin: "{{ admin_email }}"
+# prosody_virtual_domain: "{{ domain }}"
+# prosody_accounts:
+#   - name: "{{ main_user_name }}"
+#     password: TODO

roles/xmpp/defaults/main.yml

+prosody_admin: "{{ admin_email }}"
+prosody_virtual_domain: "{{ domain }}"
+prosody_accounts: []