externalize variable for filemgmt.jboss.org so it can be swapped for IP (performance improvement)

Recently, filemgmt moved IP addresses and any hardcoded refs to the old place are taking FOREVER to write files. So we need to parameterize this... or go back to using the domain name. But either way, this shouldn't be hardcoded in places like this:

https://github.com/jbosstools/jbosstools-build-sites/blob/master/aggregate/central-site/pom.xml#L169

So the variable ought to be moved into the parent pom for global and local override options.