JBIDE-13599 - externalize Central site URL

   Documentation added where is needed.
   DiscoveryConnector moved to JBossDiscoveryConnector to avoid confusion
and ClassCastExceptions.
   siteUrl resolution is now only in  JBossDiscoveryConnector.validate() method
   Ancestor of ExpressionResolutionException changed to RuntimeException and
tests for resolver changed to reflect this cahnge.
   DiscoveryTest reflection method are replaced with direct method calls
JBIDE-13599 - externalize Central site URL