Add "tolerations" parameter in Kubernetes context

Add "tolerations" parameter in Kubernetes context

This new parameter, "torelations", is applied to pods and allow (but not
require) the pods to schedule onto node with matching tains [1].

Example of "tolerations" definition in a Kubernetes pod:

  spec:
    containers: [...]
    tolerations:
      - key: <key defined in the taint>
        value: <key value to match with the taint>
        effect: <effect in case of match>
        operator: <matching operator>
      - key: ...

Example of "tolerations" definition in a Yardstick test case:

  context:
    type: Kubernetes
    servers:
      host:
        containers: [...]
        tolerations:
          - key: ...
            value: ...
            effect: ...
            operator: ...

NOTE: if any toleration is defined, a default one will be applied in
order to allow any replication controller to create the pods in any
Kubernetes node. This default toleration is defined as:

  spec:
    tolerations:
      - operator: "Exists"

[1] https://kubernetes.io/docs/concepts/configuration/taint-and-toleration/

JIRA: YARDSTICK-1254

Change-Id: I32fb9c7086b4218c323218738057f634eb6ffff4
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>