use x509 for postfix ca and fix names for cert+key

smtpd_tls_CAfile = /etc/leap/files/ca/client_ca.crt
smtpd_tls_cert_file = /etc/x509/certs/single1.crt
smtpd_tls_key_file = /etc/x509/keys/single1.key

single1.* should be postfix.*

(from redmine: created on 2013-09-13, closed on 2013-09-19, blocks #3874 (closed))