Improve kola argument processing logic for platform specific options

# Feature Request #

If a platform specific option is passed to kola, it should default to that platform. For example if `--aws-ami` but the platform is not set, kola should default to the `aws` platform.

Example from (now removed): https://github.com/coreos/coreos-assembler/blob/0a09bd74a376fb5bda9142d3da1ee10b7b34a2f3/mantle/cmd/kola/options.go#L180