Add support for scos build testing

Supported:
- scos-
- scos-coreos
- rhcos-
- rhcos-coreos

See openshift/os#773
Fix issue
```
$ cosa kola run ostree.basic
        harness.go:1443: mach.Start() failed: machine "ea43750e-e04b-4632-b966-caff659a5214" failed basic checks: not a supported instance: scos-coreos
```

mantle/platform/platform.go

@@ -493,7 +493,7 @@ func CheckMachine(ctx context.Context, m Machine) error {
 	switch string(out) {
 	case `fedora-coreos`:
 		distribution = "fcos"
-	case `centos-coreos`, `rhcos-`:
+	case `scos-`, `scos-coreos`, `rhcos-`, `rhcos-coreos`:
 		distribution = "rhcos"
 	default:
 		return fmt.Errorf("not a supported instance: %v", string(out))