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://wiki.jenkins-ci.org/display/JENKINS/Basic+guide+to+Jelly+usage+in+Jenkins#BasicguidetoJellyusageinJenkins-PredefinedURLs). Newer versions of Jenkins changed `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 `app.rootUrl`

If you are not ultimately inside `<l:layout>`, you can also use

    <j:new var="h" className="hudson.Functions"/>
    ${h.initPageVariables(context)}

to initialize standard variables such as `rootURL`.