Import auth providers to support GKE, etc.

[jbarrick-mesosphere]

What type of PR is this?

/component kudoctl
/kind bug

What this PR does / why we need it:

We need to import the auth providers in order to support authentication with GKE and other providers that require built in auth.

Before:

➜  test-infra git:(prow) ✗ go run github.com/kudobuilder/kudo/cmd/kubectl-kudo get instances
Error: creating kudo client: No Auth Provider found for name "gcp"
Usage:
  kubectl-kudo get instances [flags]

Flags:
  -h, --help               help for get
      --namespace string   The namespace where the operator watches for changes. (default "default")

Global Flags:
      --kubeconfig string   Path to your Kubernetes configuration file (default "/home/justin/.kube/config")

exit status 255
➜  test-infra git:(prow) ✗

After:

➜  test-infra git:(prow) ✗ go run github.com/kudobuilder/kudo/cmd/kubectl-kudo get instances                   
Error: creating kudo client: operators: the server could not find the requested resource (get operators.kudo.dev)
Usage:
  kubectl-kudo get instances [flags]

Flags:
  -h, --help               help for get
      --namespace string   The namespace where the operator watches for changes. (default "default")

Global Flags:
      --kubeconfig string   Path to your Kubernetes configuration file (default "/home/justin/.kube/config")

exit status 255
➜  test-infra git:(prow) ✗ 

Special notes for your reviewer:

I have no idea where this stuff is documented... I just have had someone submit a PR to do the same thing on a different one of my projects: oliviabarrick/go-k8s-portforward#1

Does this PR introduce a user-facing change?:

NONE

[jbarrick-mesosphere]

pkg/kudoctl/util/kudo/kudo.go:12:2: a blank import should be only in a main or test package, or have a comment justifying it

Okay, fair enough, linter.