Add 'kubeconfig_file' param into the out params

[superbrothers]

The kubeconfig_file is a new out param, which is the path of
kubeconfig file. With this param, we can use any remote kubeconfig file
which is fetched by other resources like s3-resource.

This param has priority over the kubeconfig of source configuration.

Example: Use a remote kubeconfig file fetched by s3-resource

resources:
- name: k8s-prod
  type: kubernetes

- name: kubeconfig-file
  type: s3
  source:
    bucket: mybucket
    versioned_file: config
    access_key_id: ((s3-access-key))
    secret_access_key: ((s3-secret))

- name: my-app
  type: git
  source:
    ...

jobs:
- name: k8s-deploy-prod
  plan:
  - aggregate:
    - get: my-app
      trigger: true
    - get: kubeconfig-file
  - put: k8s-prod
    params:
      kubectl: apply -f my-app/k8s -f my-app/k8s/production
      wait_until_ready_selector: app=myapp
      kubeconfig_file: kubeconfig-file/config # <--- A new param here

Closes #16

[tksm]

LGTM 👍

[inajob]

LGTM