Add support for variables in agent declaration in pipelines

[gayanper]

Add support for following style pipeline testing

def agent = ""
pipeline {
     stages {
           stage("prep") {
                  steps {
                     script {
                       agentLabel = "windows"
                    }
           }

          stage ("build") {
               agent {
                    label "${agentLabel}"
               }
          }
     }
}

Right now the agent is evaluated before the "prep" stage is done executing.