Adds Vtep and VtepBinding to Python Client / CLI.

- Adds Vtep and VtepBinding, wrapper classes for corresponding Midonet
  API DTOs.
- At the moment, all it can do is:
  1) to create a VTEP,
  2) list VTEPS,
  3) add a VTEP binding.
- Also implemented CLI.

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()

Sample CLI inputs:
  create vtep management-ip 119.15.112.22 management-port 6633 name 'my cli vtep'
  vtep management-ip 119.15.112.22 add vtep-binding port-name in1 vlan-id 127 network-id 9fdab92a-07ff-4616-95f4-ce53cc3f2d3f

Fix MN-1586