first cut of custom gerrit labels

* new format for the callback results (review & summary)
* test cases were renamed to _legacy (and they pass)
slightly refactor status_gerrit unit test cases

* dropped unnecessary lambda
* re-structured the code so test cases for the new style review/summary
  callbacks could re-use the same test harness
implement test cases for new style GerritStatus callbacks
fix missed pep8 compliance
move GerritStatusPush description before the example
update documentation for the change

* move parameter description to cfg-statustargets.rst
* update examples/{git,repo}_gerrit.cfg to match the expected callback result
* user git_gerrit.cfg as a source for examples, instead of putting a huge
  example text in the cfg-statustargets.rst
* update release notes
Merge sa2ajj/buildbot:gerrit-labels (PR #1175)

+autopep8