OpenDKIM setup. (#196)

* OpenDKIM setup.

see #173

* Fixed unit tests.

* Fixed mysql syntax.

modoboa_installer/scripts/files/opendkim/dkim_view_mysql.sql

@@ -0,0 +1,5 @@
+CREATE OR REPLACE VIEW dkim AS (
+  SELECT id, name as domain_name, dkim_private_key_path AS private_key_path,
+          dkim_key_selector AS selector
+  FROM admin_domain WHERE enable_dkim=1
+);