Ensure balancer config is written to correct location

Balancer configuration is written to wrong directory if custom confd_dir
is passed to Apache.

Change Concat resource to use `$::apache::confd_dir` rather than
`$::apache::params::confd_dir`.

This is required for httpd24 (RHEL/CentOS 6.5) which uses
`/opt/rh/httpd24/root/etc` for configuration.