rgw: fix handling RGWUserInfo::system in RGWHandler_REST_SWIFT.

Before this patch the flag was wrongly handled in the Swift API
implementation. In rare conditions this might result in setting
req_state::system_request.

This may happen only if both of those conditions are fulfilled:
 * RadosGW is running in a multi-site configuration (at least
   one user with the system flag turned on is present),
 * the "rgw_swift_account_in_url" configurable has been switched
   to true. The value is false by default and our documentation
   doesn't actually mention about the option.

The issue doesn't affect Jewel nor any previous release.

Fixes: http://tracker.ceph.com/issues/18476
Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>

src/rgw/rgw_rest_swift.cc

@@ -2115,7 +2115,7 @@ int RGWHandler_REST_SWIFT::authorize()
     }
 
     /* FIXME(rzarzynski): move into separated RGWAuthApplier decorator. */
-    if (s->user->system) {
+    if (s->user->system && s->auth_identity->is_owner_of(s->user->user_id)) {
       s->system_request = true;
       ldout(s->cct, 20) << "system request over Swift API" << dendl;