externalize Central site URL into a commandline property so that testing or mirroring is easier

As discussed in https://issues.jboss.org/browse/JBDS-2469?focusedCommentId=12755106&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12755106 testing Central is tricky if it's not properly bootstrapped, and bootstrapping is hard when we're on an early Alpha and don't want bits to be public before they've passed QE.

A better approach than having the update site URL used in Central's discovery plugin.xml hardcoded into that file would be to have it read from a Preference in Eclipse or JBDS. This would allow it to be overwritten/overridden should a user want to test installation from a different URL than the default value.

This might even make it possible to have the same discovery plugin used for JBT and JBDS (assuming the list of connectors were the same, and certification was to appear for both instances) simply by having the preference changed to a different default URL when installing JBT or JBDS BYOE.