topology: Add REST API

1. run topology/switches and app/rest_topology
$ ./bin/ryu-manager --verbose --observe-links ryu/topology/switches.py ryu/app/rest_topology.py

2. You can get topology json
$ curl http://127.0.0.1:8080/v1.0/topology/links
[{"src": {"hw_addr": "06:96:65:51:f8:ff", "name": "s2-eth3", "port_no": "00000003", "dpid": "0000000000000002"}, "dst": {"hw_addr": "4a:ed:13:ea:b7:e0", "name": "s1-eth1", "port_no": "00000001", "dpid": "0000000000000001"}}, {"src": {"hw_addr": "4a:ed:13:ea:b7:e0", "name": "s1-eth1", "port_no": "00000001", "dpid": "0000000000000001"}, "dst": {"hw_addr": "06:96:65:51:f8:ff", "name": "s2-eth3", "port_no": "00000003", "dpid": "0000000000000002"}}, {"src": {"hw_addr": "1e:7b:0b:fc:0a:2f", "name": "s1-eth2", "port_no": "00000002", "dpid": "0000000000000001"}, "dst": {"hw_addr": "4e:62:98:6f:16:57", "name": "s3-eth3", "port_no": "00000003", "dpid": "0000000000000003"}}, {"src": {"hw_addr": "4e:62:98:6f:16:57", "name": "s3-eth3", "port_no": "00000003", "dpid": "0000000000000003"}, "dst": {"hw_addr": "1e:7b:0b:fc:0a:2f", "name": "s1-eth2", "port_no": "00000002", "dpid": "0000000000000001"}}]

$ curl http://127.0.0.1:8080/v1.0/topology/switches
[{"ports": [{"hw_addr": "4a:ed:13:ea:b7:e0", "name": "s1-eth1", "port_no": "00000001", "dpid": "0000000000000001"}, {"hw_addr": "1e:7b:0b:fc:0a:2f", "name": "s1-eth2", "port_no": "00000002", "dpid": "0000000000000001"}], "dpid": "0000000000000001"}, {"ports": [{"hw_addr": "be:3b:8d:77:14:a5", "name": "s2-eth1", "port_no": "00000001", "dpid": "0000000000000002"}, {"hw_addr": "1e:bf:ec:8b:c2:5b", "name": "s2-eth2", "port_no": "00000002", "dpid": "0000000000000002"}, {"hw_addr": "06:96:65:51:f8:ff", "name": "s2-eth3", "port_no": "00000003", "dpid": "0000000000000002"}], "dpid": "0000000000000002"}, {"ports": [{"hw_addr": "52:a2:9a:7a:a8:a1", "name": "s3-eth1", "port_no": "00000001", "dpid": "0000000000000003"}, {"hw_addr": "62:25:ff:a5:a1:1a", "name": "s3-eth2", "port_no": "00000002", "dpid": "0000000000000003"}, {"hw_addr": "4e:62:98:6f:16:57", "name": "s3-eth3", "port_no": "00000003", "dpid": "0000000000000003"}], "dpid": "0000000000000003"}]
ryu-client: support Topology REST API
ryu-client: print http response