openvpn can be configurd to use username+password authentication using an LDAP backend - there are tutorials for that on this forum. However, what exactly do you want your setup to do if LDAP fails: fail over to a second setup that does NOT require a username+password? should the LDAP setup be able to accept users WITHOUT a certificate?
It is NOT possible to do something like
- connect using username+password, authenticate against the LDAP server
- if that fails, use a certificate
that would require 2 openvpn server setups, and your client configs would have to fail over from one to the other.