Support scheduling gateways with physical network

Currently, gateway scheduler supports scheduling with all chassis. In large scale deployment and for VLAN or flat external network, it's very possible only part of chassis have links to the outer world, thus scheduling gateway with physical network is a essential requirement.
This requirement contains scheduling for chassis events and DB sync.
Based on a performance consideration, I prefer to save physical network to lrp external_ids. 1st, scheduling for chassis events may be a frequent action, getting many ports and networks from neutron DB will become a heavy burden, and 2nd, provider field in network can not be changed after created, so we only need to consider lrp external_ids migration rather than network changes.
This requirement and it's test case will be implemented in several days in PIKE cycle.