[k8s-slave] Running function can potentially return values

We can expect reutrning values from function which is running
on k8s cluster

Example:
  def artifacts = common.runOnKubernetes([
    function : this.&buildCalicoContainers,
    jnlpImg  : jnlpSlaveImg,
    slaveImg : jenkinsSlaveImg
  ])
  // run system test
  node ('calico'){
    stage ("Run system tests") {
       build job: 'calico.system-test.deploy', propagate: true, wait: true, parameters:
        [
            [$class: 'StringParameterValue', name: 'CALICO_NODE_IMAGE_REPO', value: artifacts["CALICO_NODE_IMAGE_REPO"]],
            [$class: 'StringParameterValue', name: 'CALICOCTL_IMAGE_REPO', value: artifacts["CALICOCTL_IMAGE_REPO"]],
            [$class: 'StringParameterValue', name: 'CALICO_VERSION', value: artifacts["CALICO_VERSION"]],
            [$class: 'StringParameterValue', name: 'MCP_BRANCH', value: 'mcp'],
        ]
    }
  }

Change-Id: I058537700e0bec06d9e14b422d452e2e7eb45e23
diff --git a/src/com/mirantis/mcp/Common.groovy b/src/com/mirantis/mcp/Common.groovy
index 00f8bfc..cc48fdc 100644
--- a/src/com/mirantis/mcp/Common.groovy
+++ b/src/com/mirantis/mcp/Common.groovy
@@ -110,7 +110,7 @@
       ) {
       node(lbl){
         container('k8s-slave') {
-          toRun()
+          return toRun()
         }
       }
     }