[services] remove swss from WantedBy for nat service (#4991)

Otherwise, it may cause issues for warm restarts, warm reboot.
Warm restart of swss will start nat which is not expected for warm
restart. Also it is observed that during warm-reboot script execution
nat container gets started after it was killed. This causes removal of
nat dump generated by nat previously:

A check [ -f /host/warmboot/nat/nat_entries.dump ] || echo "NAT dump
does not exists" was added right before kexec:

```
Fri Jul 17 10:47:16 UTC 2020 Prepare MLNX ASIC to fastfast-reboot:
install new FW if required
Fri Jul 17 10:47:18 UTC 2020 Pausing orchagent ...
Fri Jul 17 10:47:18 UTC 2020 Stopping nat ...
Fri Jul 17 10:47:18 UTC 2020 Stopped nat ...
Fri Jul 17 10:47:18 UTC 2020 Stopping radv ...
Fri Jul 17 10:47:19 UTC 2020 Stopping bgp ...
Fri Jul 17 10:47:19 UTC 2020 Stopped bgp ...
Fri Jul 17 10:47:21 UTC 2020 Initialize pre-shutdown ...
Fri Jul 17 10:47:21 UTC 2020 Requesting pre-shutdown ...
Fri Jul 17 10:47:22 UTC 2020 Waiting for pre-shutdown ...
Fri Jul 17 10:47:24 UTC 2020 Pre-shutdown succeeded ...
Fri Jul 17 10:47:24 UTC 2020 Backing up database ...
Fri Jul 17 10:47:25 UTC 2020 Stopping teamd ...
Fri Jul 17 10:47:25 UTC 2020 Stopped teamd ...
Fri Jul 17 10:47:25 UTC 2020 Stopping syncd ...
Fri Jul 17 10:47:35 UTC 2020 Stopped syncd ...
Fri Jul 17 10:47:35 UTC 2020 Stopping all remaining containers ...
Warning: Stopping telemetry.service, but it can still be activated by:
  telemetry.timer
Fri Jul 17 10:47:37 UTC 2020 Stopped all remaining containers ...
NAT dump does not exists
Fri Jul 17 10:47:39 UTC 2020 Rebooting with /sbin/kexec -e to
SONiC-OS-201911.140-08245093 ...
```

With this change, executed warm-reboot 10 times without hitting this
issue, while without this change the issue is easily reproducible almost
every warm-reboot run.

Signed-off-by: Stepan Blyschak <stepanb@mellanox.com>

This commit is contained in: