move next_thrift_port to class P4Switch

[poohdang]

I want to start a network that has several switches with simple_switch, and load p4 program by setting 'program' option in topology.json:

···
    "switches": {
        "s1": { "program": "build/basic.json",
                "cli_input" : "linear-topo/s1-cli.txt" },
        "s2": { "program": "build/basic.json",
                "cli_input" : "linear-topo/s2-cli.txt" }
    },
    "links": [
        ["h1", "s1-p1"], ["h2", "s2-p1"], ["s1-p2", "s2-p2"]    ]
···

However, both thrift port of s1 and s2 will be set to 9090, as the function configureP4Switch in run_exercise.py does not return the same object of class ConfiguredP4Switch .

I move the next_thrift_port property to class P4Switch, so that the thrift port number will be incremented just like grpc port.