Allow override of cluster builder namespace from flag

[balopat]

Kaniko pods are always executed in build.cluster.namespace. This made integration testing awkward in #2734 (comment) - skaffold build -n <namespace> should honor the namespace setting, but maybe we need a separate flag for that - build-namespace. We should think through the implications.

[andrskom]

Hi! I would like to help with this issue.
What is the best way in your opinion, should I use Namespace from SkaffoldOptions struct or create a new option.

The second one looks better in my opinion, because we can separate the process of the building and the process of the deployment.

Please let me know your decision.

[briandealwis]

@andrskom we missed your reply. If you're still interested, I think @balopat's approach of creating a new BuildNamespace field in the SkaffoldOptions is still best as using the Namespace could result in confusion with skaffold dev where we both build and deploy.