Working mod_authnz_ldap support on Debian/Ubuntu

Remove the `authnz_ldap` key from $mod_packages in `apache::params` so
an installation of the non-existent package `libapache2-mod-authz-ldap`
is not attempted.

Properly manage the `authnz_ldap.conf` file template and set the
`LDAPVerifyServerCert` directive according to $verifyServerCert
in `apache::mod::authnz_ldap`.

This fixes only the superficial problems in issue #494. Better support
for mod_ldap and mod_authnz_ldap is required, as discussed there.