Add a disabled-contexts feature for mod_cluster

Currently, if you want contexts disabled at start up, you can only have them all disabled with auto-enable-contexts="false". In some cases, you may want only specific apps disabled at start up. So it would be helpful to have a disabled-contexts parameter. This would accept a list of contexts similar to excluded-contexts, but the listed contexts in disabled-contexts would designate what contexts must be added to the balancer in a disabled state.