Allow running altool action in validate mode

[mgrebenets]

Sometimes it's desirable to validate the app only, but not to upload it.
The difference in actual altool command is --upload-app vs --validate-app, however the wrapper hard-codes it to use --upload-app.

Maybe having altool_validate and altool_upload actions would be an option?
Or having an "action" parameter to altool defaulted to :upload?
E.g.

altool(action: :upload, ...) and altool(action: :validate)...