add 'policy_based_routing_doc_procedure' test

Add test, that covers use case from documentation example.

It creates 4 namespaces (4 different networks), two are providers, one is for internal workstations and one for servers. The test cretaes policy-based routing rules in NM connections, and then checks, that everything works (servers use privder A, workstations provider B).

To make this work, eth0 has to be disabled, and so there is no access to internet. The fix is to create veths in providers' namespaces and assign them the same address (172.20.20.20). Then check tcpdump log, whether there is the traffic in correct namespace.

@RunTests:policy_based_routing_doc_procedure