Make the "port forwarding delete" command idempotent

[larsks]

Previously, attempting to delete a port forwarding that does not exist
would fail with:

$ uv run openstack  esi port forwarding delete 172.17.1.106 128.31.20.118 -p 80
'could not find port forwarding matching 80:80/tcp'
$ echo $?
1

With this change, we log a warning but do not consider it an error:

$ uv run openstack  esi port forwarding delete 172.17.1.106 128.31.20.118 -p 80
port forwarding matching 80:80/tcp does not exist
$ echo $?
0