remove stale pureftpd::config_ldap docs
remove unused class parameters/docs from pureftpd::config_{mysql,pgsql}
mv pureftpd::config_{ldap,mysql,pgsql} -> pureftpd::config::{ldap,mysql,pgsql}
refactor pureftpd::config similar to pureftpd::config::ldap

to allow the same erb template to be shared between pureftpd::config and
pureftpd::config::ldap