Ignore any error returned from ip neigh flush

In the test_po_update test case, one of the things done there is to
remove an IP address from a port channel interface. As part of that, the
current handling for that issues a `ip neigh flush dev ...` command,
added in sonic-net/sonic-buildimage#606, presumably to remove old
neighbor entries that would no longer be valid. I would think that
the kernel would automatically do this, but maybe it didn't back then;
I'm not sure if there's been a behavior change here since then.

In some cases, this command is returning an error, saying "Failed to
send flush request: No such file or directory". I'm not sure why this
is; maybe when iproute2 is going through the list of neighbors, some
neighbor entry was there, but then by the time it issued the deletion
request, that neighbor entry was removed by the kernel since the IP
address was removed. Either way, I don't believe a failure here is
critical.

Therefore, ignore any failures from running this command.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

config/main.py

@@ -4702,7 +4702,7 @@ def remove(ctx, interface_name, ip_addr):
         command = ['sudo', 'ip', 'netns', 'exec', str(ctx.obj['namespace']), 'ip', 'neigh', 'flush', 'dev', str(interface_name), str(ip_address)]
     else:
         command = ['ip', 'neigh', 'flush', 'dev', str(interface_name), str(ip_address)]
-    clicommon.run_command(command)
+    clicommon.run_command(command, ignore_error=True)
 
 #
 # 'loopback-action' subcommand