Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "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>