gradle: Allow bnd_jar to be a URL

The bnd_jar property can now be either a relative path to a file in the
workspace or a URL. If the URL is not a file: URL, then it is downloaded
into a local file in cnf/cache which is then used as the bnd jar.
gradle: Use latest bnd jar from CloudBees to build bnd

This uses the latest bnd.master built jar to build bnd.
Dog fooding...
gradle: Use BndPlugin.gradle from bnd jar

Since we now build with a version of bnd jar which includes the BndPlugin
gradle script, we now load the script from the bnd jar instead of the
workspace.