mgr/dashboard: Only delete removed gateways
PR https://github.com/ceph/ceph-iscsi/pull/51 introduces the support for deleting a single gateway.
This means that, instead of deleting the target (and all gateways) when "one" gateway is removed, we can simply delete the removed gateway and left the target and remaining gateways untouched.
For instance, deleting `node2` from the following config:
iqn.2003-01.com.redhat.iscsi-gw:iscsi-igw |_ node1 192.168.100.201 |_ node2 192.168.100.202 |_ node3 192.168.100.203
will perform the following operations:
- delete iqn.2003-01.com.redhat.iscsi-gw:iscsi-igw
- create iqn.2003-01.com.redhat.iscsi-gw:iscsi-igw
- create node1 and node3
- only delete node3