Skaffold file output flags are inconsistent

Skaffold supports writing a few different run results to files for use in other commands or k8s-related processes. However, these files all have different flag names, making it difficult to know which one to use for which command.

writing the built artifact results from `skaffold build` - `--file-output`
consuming this generated file in `skaffold deploy` - `--artifacts`, or `-a`
outputting templated manifests from `skaffold render` - `--output`, or `-o`

We should decide on a common flag name and just use that everywhere, or at least for `build` and `render` where the flag is actually specifying an output.