Add 'kubectl' transport support for CLI client

[dkliban]

pulp-smash currently supports two types of transports for the CLI client: local and ssh. When Pulp 3 is deployed using Kubernetes, neither the local nor the ssh transport can be used to execute commands inside the container. In these instances, 'kubectl' needs to be used to execute a command. The kubectl command needs to be executed locally in the following manner:

kubectl exec -it pulp-api-<pod-id> /some/command --with-argument

kubectl needs to be used to discover the pulp-api pod name. This should occur once when the CLI client is instantiated. This information is available by running the following:

$ kubectl get pods
NAME                                    READY   STATUS    RESTARTS   AGE
postgres-56bc7d8cf7-z7z27               1/1     Running   0          3h47m
pulp-api-54b6bb8599-vg4xq               1/1     Running   0          3h47m
pulp-content-6b4c4998cf-2bvnq           1/1     Running   0          3h47m
pulp-content-6b4c4998cf-p6gp8           1/1     Running   0          3h47m
pulp-operator-dfcd54ff9-5qh6j           2/2     Running   0          3h48m
pulp-resource-manager-fcb4668dd-8n5rp   1/1     Running   0          3h47m
pulp-worker-69d98df488-6dpxm            1/1     Running   0          3h47m
pulp-worker-69d98df488-74fsr            1/1     Running   0          3h47m
redis-59b564fc74-czvss                  1/1     Running   0          3h47m

pulp-smash users will be able to specify the following config to run tests against pulp 3 on kubernetes:

{
    "pulp": {
        "auth": ["admin", "admin"],
        "selinux enabled": false,
        "version": "3"
    },
    "hosts": [
        {
            "hostname": "localhost",
            "roles": {
                "api": {"port": 24817, "scheme": "http", "service": "nginx"},
                "content": {"port": 24816, "scheme": "http", "service": "pulp_content_app"},
                "pulp resource manager": {},
                "pulp workers": {},
                "redis": {},
                "shell": {"transport": "kubectl"}
            }
        }
    ]
}