[FIXED JENKINS-25266] added support for the forcePush option in the git publisher

This pull request adds support for the forcePush option in the Git publisher section (since [Git Plugin](https://wiki.jenkins-ci.org/display/JENKINS/Git+Plugin) 2.2.6).

The syntax is: 

    job {
      ...
      publishers {
        git {
          forcePush()
          ...
        }
      }
    }