RSS: Reuse SNAT IPs until port range is exhausted

[jgallagher]

Prior to this PR, RSS would step both the IP and the port range when assigning SNAT IPs to boundary NTP zones. E.g., on a4x2:

            "snat_cfg": {
              "ip": "198.51.100.25",
              "first_port": 0,
              "last_port": 16383
            }
            "snat_cfg": {
              "ip": "198.51.100.26",
              "first_port": 16384,
              "last_port": 32767
            }

After the change, we reuse the IP and only step the ports:

            "snat_cfg": {
              "ip": "198.51.100.25",
              "first_port": 0,
              "last_port": 16383
            }
            "snat_cfg": {
              "ip": "198.51.100.25",
              "first_port": 16384,
              "last_port": 32767
            }

I confirmed both boundary NTP zones still have external connectivity, as expected.

I believe the code here already intended to do this, but accidentally never assigned self.next_snat_ip, so always allocated a new IP.

[luqmana]

Nice catch, thanks!