For the OpenShift and Kubernetes drivers, allow passing env vars to the
Pod nodes via their label config.
It is not possible to set persistent env vars in containers on run time
because there is no login shell available. Thus, we need to pass in any
env vars during node launch. This allows to set, e.g., ``http_proxy``
variables.
The env vars are passed as a list of dicts with ``name`` and ``value``
fields as per the k8s Pod YAML schema. [1]
```
- name: pod-fedora
type: pod
image: docker.io/fedora:28
env:
- name: foo
value: bar
```
[1] https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/
Change-Id: Ibbd9222fcd8f7dc5be227e7f5c8d8772a4c594e2
b76a0f458e