Ensure balancer config is written to correct location

Concat resource for balancer configuration uses
`$::apache::params::confd_dir` rather than the value passed to Apache
class (i.e. `$::apache::confd_dir`).

This is required to use balancer resources with httpd24 (RHEL/CentOS
6.5) which relocates configuration under `/opt/rh/httpd24/root/etc`.