JBDS-2674 replace default unified target URL on VPN-only internal serverhttp://www.qa.jboss.com/binaries/RHDS/targetplatforms/jbdevstudiotarget/ with public, external server http://download.jboss.org/jbosstools/targetplatforms/jbdevstudiotarget/
backport changes in 4.30.x branch to externalize targetRepositoryUrl and validate-target-platform.failOnError properties
update README with new instructions for updating/validating TP; add doc about -Dvalidate-target-platform.failOnError and -Dmirror-target-to-repo.includeSources