Allow default dev cleanup mode to be set in skaffold.yaml

Currently skaffold dev defaults to cleanup=true.

There are occasions when I'd like to modify this behavior for all users of a specific project profile. The simplest way to do that would be a cleanup setting in skaffold.yaml.

The CLI flag would still override the setting in skaffold.yaml.

After discussion in office hours, we determined this feature might be useful but could be confusing for some use cases. Open to discussion as always :-)

Original Question: https://groups.google.com/forum/#!topic/skaffold-users/rqzKmFI-qaM