Categories optionally configured with pairs of throttled node labels.

JENKINS-19645
Pairs make each maximum applicable to nodes with matching labels only.
- Achieved by adding such Maximum Per Labeled Node pairs to categories.
Category's Maximum Concurrent Builds Per Node is superseded if matching.
Merge pull request #7 from marco-miller/labels

Categories optionally configured with pairs of throttled node labels.
[maven-release-plugin] prepare release throttle-concurrents-1.8
[maven-release-plugin] prepare for next development iteration