[FIXED JENKINS-8990] Configurable ping interval

This lets you configure the ping interval for slaves via a
system property. (hudson.slaves.ChannelPinger.pingInterval)

The default ping interval has been lowered from 10 to 5 minutes.

It also moves the ping setup logic into a ComputerListener
(out of the JNLP slave Engine class). As a side effect, all
slaves will now have a ping instead of just JNLP slaves.