rgw: RGWPeriod::reflect() sets first zonegroup as default

if RGWPeriod::reflect() is called right after a 'realm pull', it will
create the zonegroup objects but not set any as default.  so the next
time RGWRados is initialized, create_defaults() will add a new 'default'
zonegroup and set it as the master.  this causes a 'period update' to
find multiple master zonegroups and fail with EINVAL

reflect() now calls set_as_default(exclusive=true) on each zonegroup to
guarantee that the first zonegroup is made default if no default is set

Signed-off-by: Casey Bodley <cbodley@redhat.com>

src/rgw/rgw_rados.cc

@@ -1204,6 +1204,8 @@ int RGWPeriod::reflect()
       ldout(cct, 0) << "ERROR: failed to store zonegroup info for zonegroup=" << iter.first << ": " << cpp_strerror(-r) << dendl;
       return r;
     }
+    // set as default if no default exists
+    zg.set_as_default(true);
   }
   return 0;
 }