mgr/dashboard: fix host form issues

mgr/dashboard: fix host form issues

Addressing the review comments in https://github.com/ceph/ceph/pull/60355#pullrequestreview-2391335490

Issues fixing
 - cluster expansion host form not closing form when submitting it
 - cluster expansion host form changing the location when closing it
 - put the helper text inside helper component since its longer
 - maintenance field should be hidden in cluster expansion form since
   its not a valid option while expanding a cluster. We already add host
   in _no_schedule

Fixes: https://tracker.ceph.com/issues/69020
Signed-off-by: Nizamudeen A <nia@redhat.com>