Run button parameterized builds

Fixes issues relating to the run button for parameterized builds
https://issues.jenkins-ci.org/browse/JENKINS-20841
https://issues.jenkins-ci.org/browse/JENKINS-25427

Also switched to using app.rootUrl for the `fillDialog` method as recommended by the jelly guidelines (https://issues.jenkins-ci.org/browse/JENKINS-25427 - See Predefined URLs). Newer versions of Jenkins change rootUrl to be `/jenkins` and when the extra `/` was added by `fillDialog`, the URL did not work. Jenkins will automatically add the trailing slash to the `root.appUrl`