mgr/dashboard: Fix for incorrect validation in rgw user form

mgr/dashboard: Fix for incorrect validation in rgw user form

The rgw users create form doesnt validate the username correctly if the username is a tenated one. For eg. Consider there is a user called tenate$sample. Now I am trying to create another user and I entered tenate$sample as username. But it doesn't async validate the username as existing. Instead it just shows the green tick and once the submit button is clicked it'll show the user as existing.

Fixes: https://tracker.ceph.com/issues/48907
Signed-off-by: Nizamudeen A <nia@redhat.com>