[nginx-ingress-controller] Add annotation to add CORS support

[aledbf]

This change is

[bprashanth]

LGTM after the nit fix

[bprashanth]

please fix comment

[aledbf]

done

[k8s-github-robot]

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[aledbf]

I signed it

[k8s-github-robot]

Automatic merge from submit-queue

[vinay-g]

@aledbf - can you give an example on how to enable cors in ingress, I added annotation ingress.kubernetes.io/enable-cors: "true" but could not get this work. I tried this using nginx-ingress-controller:0.8.3

[aledbf]

@vinay-g this feature is not available in 0.8.3. Next release