Fix JENKINS-25676 (and also JENKINS-25222)

Instead of going with an server specific solution, re-use Smack's PingManager for all connections.

Right now it's not possible to change the server ping interval. This feature can be added in a later version. Right now the change should address JENKINS-25676 and JENKINS-25222, but would be great if the affected users could confirm that.
I guess my main point is that PingManager instance are not immutable, which is one main reason to use the builder pattern. There are situations where the ping interval should be adopted. Maybe the method name could be changed (although no good idea for a new name comes to my mind right now). Or the javadoc could be made more explicit about this.