cmd: added new flag --skip-generation for operator-sdk add api

[yangzhares]

To skip to run k8s codegen and openapi-gen to generate deepcopy and validation spec for new CRD

Description of the change:
Added new flag --skip-generation for operator-sdk add api

Motivation for the change:
To skip to run k8s codegen and openapi-gen to generate deepcopy and validation spec for new CRD

[openshift-ci-robot]

Hi @yangzhares. Thanks for your PR.

I'm waiting for a operator-framework or openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[camilamacedo86]

/ok-to-test

[camilamacedo86]

Ref: #1878

[camilamacedo86]

• Checked that it worked as expected locally
• The change was done as suggested by @joelanford
• The change was added in the CHANGELOG as well
• Commits are squashed.

/lgtm
/approve