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.