Add "env" parameter in Kubernetes context

Add "env" parameter in Kubernetes context

This new parameter, "env", will allow the user to
automatically to add environment variables in a pod definition

Example of definition in a context:
  context:
    type: Kubernetes
      servers:
        host:
            image: ...
            commands: ...
            env:
                - <variable name>: <variable value>
                - <variable name>: <variable value>

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-1161

Change-Id: Icf372a98c090063f10c9217755519ab92ba9b0ba
Signed-off-by: John O Loughlin <john.oloughlin@intel.com>