fix firewall creation fails issue

[goldenrye]

Adjust the init order when creating firewall cube object so the parameter passed
by user can be populated, the will also avoid the crash due to object reference
before object initialization.

issue #184

[frisso]

Jianwen, just a question: any idea why this was never noticed in our tests?

[goldenrye]

Jianwen, just a question: any idea why this was never noticed in our tests?

That's because we don't have test scripts to cover those cases.

If you execute the following command with the latest master branch build system will crash,
polycubectl firewall add fw-test accept-established=on

For default chain action, polycubectl doesn't allow to set the default chain action, so you won't be able run into this issue via CLI. But using curl command you should be able to reproduce the issue:
curl -d '{"name":"fw-test","chain":[{"name":"ingress","default":"forward"},{"name":"egress","default":"forward"}]}' -H "Content-Type: application/json" -X POST http://127.0.0.1:9000/polycube/v1/firewall/fw-test/

[palexster]

/lgtm