don't bind the WTP CAPWAP socket to a specific port

RFC 5415, Sect. 3.1.:
  ...
  The CAPWAP control port at the WTP can be any port
  selected by the WTP.
  ...
  The CAPWAP data port at the WTP can be any port
  selected by the WTP.
  ...

In fact, reusing the same control port makes it impossible
for the AC to know whether the WTP restarted it's CAPWAP
session and it also breaks DTLS session state logic when
if the WTP restarts before the old DTLS session timed out.