Jetty License Update

ZooKeeper server is using Jetty (apache license, v2) for admin server (and for more things in future), but we didn't include any of Jetty's copy right / notice / license file in ZooKeeper distribution. This ticket is to figure out if Jetty license is indeed missing and if so, fix it.

Some previous discussions on Jetty license in ZOOKEEPER-2235 but Jetty somehow was not ended up in the patch.