kubeless function describe panic

[silenceshell]

Issue Ref: [Issue number related to this PR or None]

None

Description:

bug fix : kubeless function describe will panic when Containers are empty.
This happened when function is created by kubeless-ui.

func print(f kubelessApi.Function, name, output string) error {
...
    env, err := json.Marshal(f.Spec.Deployment.Spec.Template.Spec.Containers[0].Env)

The function is

$ kubectl get function xxx -o yaml
apiVersion: kubeless.io/v1beta1
kind: Function
metadata:
  clusterName: ""
  creationTimestamp: 2018-06-29T06:59:34Z
  finalizers:
  - kubeless.io/function
  generation: 1
  name: xxx
  namespace: default
  resourceVersion: "872425"
  selfLink: /apis/kubeless.io/v1beta1/namespaces/default/functions/xxx
  uid: fabbed3a-7b69-11e8-8251-6c0b84ace257
spec:
  checksum: sha256:f727da8be400e7412981aa011900905151c59c59e6ce959f3cec0f5c2bc00b8f
  deployment:
    metadata:
      creationTimestamp: null
    spec:
      strategy: {}
      template:
        metadata:
          creationTimestamp: null
        spec:
          containers: null
    status: {}
  deps: ""

And I think there is no need to use fmt.Sprintf for a string var.

TODOs:

• Ready to review
• Automated Tests
• Docs

[andresmgot]

thanks for the PR!