cover mysql use case for dict definition in dovecot 2.4

see #602

modoboa_installer/scripts/files/dovecot/2.4/conf.d/30-dict-server-mysql.conf.tpl

@@ -0,0 +1,45 @@
+##
+## Dictionary server settings
+##
+
+# Dictionary can be used to store key=value lists. This is used by several
+# plugins. The dictionary can be accessed either directly or though a
+# dictionary server. The following dict block maps dictionary names to URIs
+# when the server is used. These can then be referenced using URIs in format
+# "proxy::<name>".
+
+dict_server {
+  mysql %dbhost {
+    port = %dbport
+    dbname = %modoboa_dbname
+    user = %modoboa_dbuser
+    password = %modoboa_dbpassword
+  }
+  dict quota {
+    driver = sql
+    sql_driver = %db_driver
+    hostname = %dbhost
+
+    dict_map priv/quota/storage {
+      sql_table = admin_quota
+      username_field = username
+      value_field bytes {
+        type = uint
+      }
+    }
+
+    dict_map priv/quota/messages {
+      sql_table = admin_quota
+      username_field = username
+      value_field messages {
+        type = uint
+      }
+    }
+  }
+}
+
+quota_clone {
+  dict proxy {
+    name = quota
+  }
+}