CVE-2016-2113: s4:lib/tls: create better certificates and sign the host cert with the ca cert

The generated ca cert (in ca.pem) was completely useless,
it could be replaced by cert.pem.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11752

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
