Gerrit Trigger configuration should honor central gerrit configuration.

Formerly, build status values that where not set where hardcoded in
GerritContext, so even if you defined different default values in your
global gerrit trigger configuration (like CR +2 for sucessful builds),
they would be overriden.

This commit sets default value null and only creates
config entries, when value is not null.
Fixed CodeNarc violations
Release Notes: Jenkins-23318
Updated changed syntax for gerritTrigger

Also replace example with API example
Included changes to GerritTrigger config.
cleanup
Merge branch '23318-gerrit-default-build-values'