Add CORS support

[imishravmw]

Issue Ref: [Issue number related to this PR or None]
934
Description:
Support for CORS for HTTP endpoints is expected here. This pull request provides cors flag while creating HTTP trigger.

[PR Description]
cors flag has been added to below command
kubeless trigger http create
cors-enable flag is a boolean flag, by default it is false to be backward compatible.
if cors-enable flag is true, then one can specify cors-domain which will set that only for this cors-domain, cors will be enabled.
Tests have been added to http-trigger repo.
TODOs:

• Ready to review
• Automated Tests
• Docs

[andresmgot]

Like I said in your other PR, I would make this about adding annotations instead. You can use a StringSlice like the label flag in the function deploy command:
https://github.com/kubeless/kubeless/blob/master/cmd/kubeless/function/deploy.go#L275

[imishravmw]

@andresmgot
circleci build is failing because of following error

github.com/kubeless/kubeless/cmd/kubeless/trigger/http

cmd/kubeless/trigger/http/create.go:106:19: httpTrigger.Spec.CorsEnable undefined (type "github.com/kubeless/kubeless/vendor/github.com/kubeless/http-trigger/pkg/apis/kubeless/v1beta1".HTTPTriggerSpec has no field or method CorsEnable)

even though earlier PR has been merged and code is there in master branch, any idea ,what could be causing this failure.

[andresmgot]

ah, true, you need to update the vendor dependency. You can do that executing:

dep ensure -v -update github.com/kubeless/http-trigger

[imishravmw]

@andresmgot build has passed, thanks, you can merge this PR , and should we close CORS issues as well. there are two CORS related issue in kubeless repo.

[andresmgot]

Thanks @imishravmw !