Add support to publish OpenClover coverage report

[jackjia-ibm]

It relies on plugin: https://wiki.jenkins.io/display/JENKINS/Clover+Plugin

Example pipeline code:

  step([
    $class: 'CloverPublisher',
    cloverReportDir: 'target/site',
    cloverReportFileName: 'clover.xml',
    healthyTarget: [methodCoverage: 70, conditionalCoverage: 80, statementCoverage: 80], // optional, default is: method=70, conditional=80, statement=80
    unhealthyTarget: [methodCoverage: 50, conditionalCoverage: 50, statementCoverage: 50], // optional, default is none
    failingTarget: [methodCoverage: 0, conditionalCoverage: 0, statementCoverage: 0]     // optional, default is none
  ])