Add "securityContext" parameter in Kubernetes context

Add "securityContext" parameter in Kubernetes context

This new parameter, "securityContext", will allow the user to define the
privilege and access control settings for a pod or a container [1].

Example of "securityContext" definition in the pod (if only one container
is defined):

  context:
    type: Kubernetes
    servers:
      host:
        image: ...
        securityContext:
          runAsUser: 1000
          fsGroup: 2000 

Example of "securityContext" definition in the pod and the container (if
several containers are defined):     

  context:
    type: Kubernetes
    servers:
      host:
        securityContext:
          runAsUser: 1000
          fsGroup: 2000 
        containers:
          - image: ...
            securityContext:
              allowPrivilegeEscalation: false
          - image: ...

[1] https://kubernetes.io/docs/tasks/configure-pod-container/security-context/

JIRA: YARDSTICK-1156

Change-Id: I597a300c68cd834522a284b1cca0faa918493342
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>