[BUG] Examples problem

[RaffaeleTrani]

Describe the bug

Examples reported in https://github.com/polycube-network/polycube/tree/master/src/services/pcn-firewall/examples are not working as expected.

To Reproduce

Steps to reproduce the behavior:

1. Go to in https://github.com/polycube-network/polycube/tree/master/src/services/pcn-firewall/examples
2. Run script https://github.com/polycube-network/polycube/blob/master/src/services/pcn-firewall/examples/setup_veth.sh (works fine)
3. Try to start https://github.com/polycube-network/polycube/blob/master/src/services/pcn-firewall/examples/allow_ping.sh
4. polycubectl firewall fw ports add fw-p1 return a non-zero code.
5. As expected, the test script fails when testing. https://github.com/polycube-network/polycube/blob/master/src/services/pcn-firewall/examples/test_ping.sh

Expected behavior

It should create the expected configuration