Use the environment variables provided by the GerritCodeReview-plugin
Use the environment variables provided by the GerritCodeReview-plugin

So far the pipeline described in the Jenkinsfile queried Gerrit to get
the change metadata. By now the GerritCodeReview provides this data as
environment variables. This reduces the boilerplate code needed in the
Jenkinsfile.

Change-Id: Ibc18d7f1b350e6070e93164cea6a5ff3fd826ae3

WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit