IKEv2: don't include the KE payload in the INIT packet

Interop #1 omits KE from the request, which is rejected by the
reponder.

Interop #2 includes KE, but then gets no KE back from the responder.

Basic pluto with IKEv2 using PSK but west has been told to skip
sending the KE payload. This is to confirm the fix for CVE-2013-6467,
which affects libreswan up to 3.7 (and openswan up to at least 2.6.39)
This test crashes those older versions
