Adds Vtep and VtepBinding to Python Client.

- Adds Vtep and VtepBinding, wrapper classes for corresponding Midonet
  API DTOs.
- At the moment, all it can do is: 1) to create a VTEP, and 2) add a
  VTEP binding.

Sample code fragment)

  # Create a new VTEP
  # vtep_management_ip/port are assumed to contain right values.
  vtep = mc.add_vtep().name('My VTEP').management_ip(vtep_management_ip).management_port(vtep_management_port).create()

  # Add a new VTEP binding.
  # bridge_id is assumed to contain a correct Mido bridge ID.
  vtep_binding = vtep.add_binding().port_name('in1').network_id(bridge_id).create()