MAR-165: Deploy Diffie-Helman parameters for LDAP server in the ldap_server role:

- Not relevant for Debian Strech because of a bug in the OpenLDAP
version it ships with.
- This should allow use of DHE ciphers with LDAP server.
- Generated DH parameters only help pick one of the parameters from
RFC-7919 (based on the size of generated ones).
- Make the cipher test lists distro-specific due to differences
between supported algorithms in respective GnuTLS versions.