Do not include radicale job if not needed.

see #24

modoboa_installer/scripts/files/modoboa/crontab.tpl

@@ -23,4 +23,4 @@ INSTANCE=%{instance_path}
 */5    *       *       *       *       root    $PYTHON $INSTANCE/manage.py logparser &> /dev/null
 
 # Radicale rights file
-*/2    *       *       *       *        root    $PYTHON $INSTANCE/manage.py generate_rights
+%{radicale_enabled}*/2    *       *       *       *        root    $PYTHON $INSTANCE/manage.py generate_rights

modoboa_installer/scripts/modoboa.py

@@ -89,9 +89,12 @@ class Modoboa(base.Installer):
     def get_template_context(self):
         """Additional variables."""
         context = super(Modoboa, self).get_template_context()
+        extensions = self.config.get("modoboa", "extensions")
+        extensions = extensions.split()
         context.update({
             "dovecot_mailboxes_owner": (
-                self.config.get("dovecot", "mailboxes_owner"))
+                self.config.get("dovecot", "mailboxes_owner")),
+            "radicale_enabled": "#" if "modoboa-radicale" in extensions else ""
         })
         return context