[rollout-operator] Replace default webhooks.selfSignedCertSecretName with fullname

[kimxogus]

Certificate secret's default name certificate is too general name, so I made it chart's fullname to match with the other resources.

Changing cert secret name doesn't break existing installations. It will recreate rollout-operator pod and cert secret will be recreated as well.

[tcp13equals2]

I have double checked that we have the correct permissions for this certificate to be re-created.

This change looks to be safe, but could we just update the values.yaml and/or README/md.gotmpl with additional documentation explaining that the operator can safely update this value if need be.

Is there a specific issue that this generic name is causing? A conflict in the namespace perhaps?

[kimxogus]

@tcp13equals2 Sorry, I was too late. I've just applied suggested changes.