[FIXED JENKINS-36715] - Invoke FindBugs during the Jenkins core build (#2458)

* Move FindBugs call to the parent POM, always run it and generate XMLs

* Update FindBugs to 3.0.4

* Filter away Stream encoding issues

* Rework FindBugs definitions according to comments from @jtnord

* Generalize configuration, findbugs:findbugs now takes it.

* Add TODOs to FindBugs excludes file

* Remove explicit failOnError

* Fix the typo spotted by @jtnord