move tarsnap to its own role

roles/common/tasks/main.yml

 - include: ssl.yml tags=ssl
 - include: ferm.yml tags=ferm
 - include: security.yml tags=security
-- include: tarsnap.yml tags=tarsnap

roles/common/vars/main.yml

 main_user_name: TODO
 admin_email: TODO@TODO.com
-tarsnap_version: 1.0.35
 encfs_password: TODO

roles/tarsnap/tasks/main.yml

+- include: tarsnap.yml tags=tarsnap

roles/tarsnap/vars/main.yml

+tarsnap_version: 1.0.35

site.yml

     - monitoring
     - owncloud
     - vpn
+    - tarsnap
+