Add "imagePullPolicy" parameter in Kubernetes container

Add "imagePullPolicy" parameter in Kubernetes container

The configuration parameter "imagePullPolicy" defines how Kubernetes
retrieves de image. By default, Kubernetes will always try to pull the
latest image from the repository [1]. Three possible arguments are accepted:
  - Always
  - IfNotPresent
  - Never

Example of Kubernetes pod definition:
  containers:
    - name: vcmts-pktgen-pod-0
      image: openretriever/yardstick
      imagePullPolicy: IfNotPresent

Example of Yardstick definition:
  context:
    name: k8syardstick
    type: Kubernetesservers:
    vnf:
      image: openretriever/yardstick
      imagePullPolicy: IfNotPresent

[1] https://kubernetes.io/docs/concepts/containers/images/

JIRA: YARDSTICK-1328

Change-Id: I83bfb01670eef22218b1affd7d37af14e32a622e
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>