Jenkinsfile: Migrate properties to declarative options/triggers.

Properties is a scripted pipeline concept and doesn't work correctly
in the context of a *declarative* pipeline.

* Jenkinsfile: Add 'ansicolor' to the list of requirements in the top
comment.
(properties): Remove.
(triggers): New directive.

Change-Id: Idceff87a005a935ff6af2931fd0ed466d0e73911