[JENKINS-14494] Flexible Publish works on Multi-configuration projects.

Implementation for [JENKINS-14494](https://issues.jenkins-ci.org/browse/JENKINS-14494).

`FlesiblePublisher` supports `MatrixAggregatable`.
Provides a wrapper for `MatrixAggregator`, and evaluates conditions before propagating to wrapped `MatrixAggregator`.

You can specify a condition for parent aggregation other than a condition for execution on each axe combinations.