fix: enable command for crd resources

[hectorj2f]

What this PR does / why we need it:

kubefedctl enable crd command was throwing a panic as reported in #1206. It is related to what @marun did here #470. I might need a bit more of context to address the TODO comment.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #
#1206
Special notes for your reviewer:

kubefedctl enable crd
customresourcedefinition.apiextensions.k8s.io/federatedcustomresourcedefinitions.types.kubefed.io created
federatedtypeconfig.core.kubefed.io/customresourcedefinitions.apiextensions.k8s.io updated in namespace kommander

I also tested to enable one of our custom resource definitions:

$ kubefedctl federate crd addons.kubeaddons.mesosphere.io

$ kubectl get federatedcustomresourcedefinitions 
NAME                              AGE
addons.kubeaddons.mesosphere.io   51s

[hectorj2f]

/assign @jimmidyson

[hectorj2f]

/assign @pmorie

[irfanurrehman]

This change seems good to me for now.
/lgtm

[pmorie]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hectorj2f, pmorie

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