RFC 7427:
Basic pluto with IKEv2 using X.509 on the initiator (west), and Strongswan on
the responder (east).

This is the reverse test of interop-ikev2-strongswan-05

By default the Authentication method now is Digital Signatures.
We currently supoort RSA with SHA2  only. RSA-SHA1 must use legacy
method.

Caveat: strongswan with authby=rsasig accepts RSA-PSS SHA2 signatures,
but responds with RSAv1.5 SHA2 signatures, which we refuse. According
to RFC 7427/8247 RSA-PSS SHA2 is MUST and RSAv1.5 SHA2 is MAY implement.
Workaround is to explicitly configure strongswan for RSA-PSS

