engine: resources: The kv resource can set un-mapped values

Previously the resource could only set values in a per-hostname
namespace, but for single, user-managed values, we'd like to be able to
control things entirely. Now this resource can do that.

etcd/world.go

@@ -128,6 +128,8 @@ func (obj *World) StrGet(ctx context.Context, namespace string) (string, error)
 }
 
 // StrSet sets the namespace value to a particular string.
+// XXX: This can overwrite another hosts value that was set with StrMapSet. Add
+// possible cryptographic signing or special namespacing to prevent such things.
 func (obj *World) StrSet(ctx context.Context, namespace, value string) error {
 	return str.SetStr(ctx, obj.Client, namespace, &value)
 }