Refactor OS type string as abstract interface

[bboreham]

This code, where we store a different string according to what we found:

wksctl/pkg/apis/wksprovider/machine/os/os.go

Lines 1230 to 1236 in 773dacf

	switch osID {
	case centOS:
	return &OS{Name: osID, runner: &sudo.Runner{Runner: sshClient}, PkgType: resource.PkgTypeRPM}, nil
	case rhel:
	return &OS{Name: osID, runner: &sudo.Runner{Runner: sshClient}, PkgType: resource.PkgTypeRHEL}, nil
	case ubuntu:
	return &OS{Name: osID, runner: &sudo.Runner{Runner: sshClient}, PkgType: resource.PkgTypeDeb}, nil

then later switch on that string:

wksctl/pkg/plan/recipe/upgrade_plans.go

Lines 24 to 25 in ede45a9

	switch pkgType {
	case resource.PkgTypeRPM, resource.PkgTypeRHEL:

wksctl/pkg/plan/recipe/install_plans.go

Lines 177 to 180 in 190eb37

	switch pkgType {
	case resource.PkgTypeRPM, resource.PkgTypeRHEL:
	// do nothing
	case resource.PkgTypeDeb:

seems like a classic case where we could instantiate an object of different concrete type behind an interface.