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.
   Test errors are fixed in DiscoveryTest
   DiscoveryTest reflection method are replaced with direct method calls

   Changes to https://github.com/jbosstools/jbosstools-central/pull/81