Add Palo Alto PANOS Support

[ryanmerolle]

Add Palo Alto PANOS Support

[xieyangxu]

Hi, it would help me test this vendor format if you could share an example PANOS configuration. You might want to use a network anonymizer before sharing, such as https://github.com/intentionet/netconan

[ryanmerolle]

To get you started, there are a number of them here https://github.com/batfish/batfish/tree/master/tests/parsing-tests/networks/unit-tests/configs/palo_alto