datapath: add key support to CAPWAP tunnel

Add tunnel key support to CAPWAP vport.  Uses the optional WSI field in a
CAPWAP header to store a 64bit key.  It can also be used without keys, in which
case it is backward compatible with the old code.  Documentation about the
WSI field format is in CAPWAP.txt.

Signed-off-by: Valient Gough <vgough@pobox.com>
[horms@verge.net.au: Various minor fixes (v4.1)]
Signed-off-by: Simon Horman <horms@verge.net.au>
[jesse: Additional parsing fixes]
Signed-off-by: Jesse Gross <jesse@nicira.com>