Add "volumes" parameter in Kubernetes context

Add "volumes" parameter in Kubernetes context

This new parameter, "volumes", will allow the user to automatically create
new volumes. Example of Kubernetes context definition:

  context:
    type: Kubernetes
    servers:
      host:
        image: ...
        commands: ...
        volumes:
          - name: volume1              # mandatory
            <volume type definition>   # mandatory

The volume type and the definition must be one of the supported ones in
Kubernetes [1].

[1] https://kubernetes.io/docs/concepts/storage/volumes/#types-of-volumes

JIRA: YARDSTICK-1152

Change-Id: I44a91c605f047de4f286407e28fb5aa2e921b00a
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>