mgr/dashboard: iSCSI targets not available if any iSCSI gateway is down
Ceph dashboard does not display iSCSI targets if any iSCSI gateway is down:
This should be improved in order to be able to:
- list iSCSI target if at least one iSCSI gateway is "up"
- edit/create an iSCSI target if all portals of that target are "up" (even if any other gateway is down)