nagios: use check_ssl for the check_ssl_ldap check

Replace check_ssl_cert with check_ssl for this simple check. This should
serve as a simple replacement for now for a small number of services
(LDAP servers).

This also removes ldap::server's $ca_name argument. While this could be
replaced with an --issuer pattern-matching check, there's really no
point: we only care if the certificate is valid, not the CA it was
issued from, and the fact we used an explicit CA filename before was a
check_cert limitation.

Change-Id: I9997f6ae773148464c150954e5ca5b1b30f56526