Updated JobGroup id now:1 was:0
[JENKINS-21119] Extend loggning to show assigned priority

Add decision-logging where the decisions taken to 
calculate the priority is more clearly shown.
Refactored to break out method getJobGroup(job)
Refactored not to use WeakHashMap
Refactored to better account for multiple starts of the same Job
[JENKINS-21119] Extend loggning to show assigned priority

Also log the name of the parent MatrixProject for a 
found MatrixConfiguration.