Download plugins to .jpi file.

This appears to mirror the current behavior of Jenkins.*  It prevents an issue where the puppet module downloads a .hpi and then Jenkins on start downloads and unpacks a .jpi and deletes the .hpi.  The next time Puppet runs it will create an empty .hpi file via the File resource and break the plugin.

* Not sure when this practice started and if an older behavior needs to be supported.  If it does there's more changes needed to get this to all work properly.