changed source/destination_ip type to ip_Address

* fixing firewall_service schema. The source and destination ip should be
of type ip_address, and not string. Added tests for same.

* Added 'id' to schema. we missed this. this was not captured in
integration_tests, because we auto-gnerate ids when missing.
It causes problem when we pass id explicitly
Merge pull request #122 from alphagov/fixing_firewall_schema

changed source/destination_ip type to ip_Address