feat(units): Add generic cloudinit path unit

Switch to triggering common user configs via a path unit. This is
particularly useful for config drive so that a config drive can be
mounted by something other than the udev triggered services, a bind
mount when running in a container for example.

units/user-cloudinit@.path

@@ -0,0 +1,5 @@
+[Unit]
+Description=Watch for a cloud-config at %f
+
+[Path]
+PathExists=%f