MXS-862: Add authenticator options and instances

Authenticators now have a similar mechanism to the `router_options`
parameter which enables configurable authentication.

The authenticators also have a new initialize entry point which is similar
to the createInstance entry point of the filters and routers. The value of
`authenticator_options` is passed as a parameter to this function. The
return vaulue of the `initialize` entry point is passed to the `create`
entry point.

server/core/test/test_mysql_users.c

@@ -213,7 +213,7 @@ int set_and_get_mysql_users_wildcards(char *username, char *hostname, char *pass
         return ret;
     }
 
-    SERV_LISTENER *port = listener_alloc(service, "testlistener", "MySQLClient", NULL, 4006, NULL, NULL);
+    SERV_LISTENER *port = listener_alloc(service, "testlistener", "MySQLClient", NULL, 4006, "MySQLAuth", NULL, NULL);
 
     dcb = dcb_alloc(DCB_ROLE_INTERNAL, port);