Verify kubernetesVersion when run 'package verify'.

[harryge00]

What this PR does / why we need it:

Fix #1266.
But currently, we do not compare kubernetesVersion with apiserver. Only the kubernetesVersion in the operator.yaml is verified.

[harryge00]

golangci-lint run
pkg/kudoctl/cmd/verify/verify.go:5: File is not `goimports`-ed with -local github.com/kudobuilder (goimports)
	"github.com/kudobuilder/kudo/pkg/version"
make: *** [Makefile:48: lint] Error 1

make lint failed. Isn't github.com/kudobuilder/kudo/pkg/version a local package?

[gerred]

What's the diff when goimports is run on the file? Is there any? CircleCI lint is passing, so not sure what's happening locally that's different.

[nfnt]

This is a great addition, we should make sure that kubernetesVersion is validated if set.
Though, currently kubernetesVersion is an optional field. I.e., we need to support cases where kubernetesVersion == "", which is also valid. Please support the case where kubernetesVersion isn't set.

So the test where kubernetesVersion == "" should result in a successful validation while something like kubernetesVersion == "invalid" should fail the validation.

[harryge00]

@nfnt When kubernetesVersion is not set, kubectl kudo install will fail with Error: unable to parse operators kubernetes version: Invalid Semantic Version. If kubernetesVersion is an optional field, should we also change the behavior of kudo install?

porridge@beczulka:~/tmp/o$ PATH=/home/porridge/Desktop/work/mesosphere/code/kudo/bin:$PATH kubectl kudo install .
Error: unable to parse operators kubernetes version: Invalid Semantic Version
porridge@beczulka:~/tmp/o$ PATH=/home/porridge/Desktop/work/mesosphere/code/kudo/bin:$PATH kubectl kudo package verify .
package is valid
porridge@beczulka:~/tmp/o$ cat operator.yaml 
apiVersion: kudo.dev/v1beta1
name: "minimal"
version: "0.1.0"
tasks:
- name: app
  kind: Apply
  spec:
    resources:
    - namespace.yaml
plans:
  deploy:
    strategy: serial
    phases:
    - name: main
      strategy: parallel
      steps:
      - name: app
        tasks:
        - app

[nfnt]

Interesting! I wasn't aware of this behavior in kubectl kudo install. Only looked at the type. Then we should keep the behavior already enforced by kubectl kudo install. No changes needed from you.