Allow mod_cluster connector address to be configurable

MODCLUSTER-168 introduced a change that broke some existing working configurations relying on using IP instead of a hostname. Also it is likely to be the cause of MODCLUSTER-91.

To have everybody happy, this needs to be made configurable, perhaps a system property that user can set to force a particular IP or hostname. It would also be very useful if there is a special value that makes the behavior same as before the changes in MODCLUSTER-168. For example setting the value to -1 would return the original behavior.