mgr/dashboard: support setting iSCSI gateways from orchestrator

mgr/dashboard: support setting iSCSI gateways from orchestrator

- Dashboard doesn't actively query iSCSI gateways from orchestrator anymore.
  Instead, Orchestrator backend invokes iscsi-gateway-add CLI after any
  iSCSI daemons are deployed.
- Add an optional argument `name` to `iscsi-gateway-add` command. If
  `name` is provided, Dashboard won't issue API request to iSCSI gateway
  to get the hostname. This is required because when orchestrator sets
  the gateway via CLI, the gateway is just spawned and might be not
  ready yet.

Fixes: https://tracker.ceph.com/issues/45163
Signed-off-by: Kiefer Chang <kiefer.chang@suse.com>