stream-ssl: Make it possible to avoid checking peer SSL certificate.

stream-ssl: Make it possible to avoid checking peer SSL certificate.

In Citrix XenServer, the hosts have SSL private keys and certificates, but
those certificates are not signed by any certificate authority.  So we
must provide a way to avoid checking certificates against a CA if we want
other OVS tools to be able to talk to XenServer hosts over SSL.  This
commit makes that possible.