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