feat(unit): Allow user to control enabling units

Fix #69 - A user may provide an `enable` attribute of a unit in their
cloud config document. If true, coreos-cloudinit will instruct systemd
to enable the associated unit. If false, the unit will not be enabled.

Fix #71 - The default enable behavior has been changed from on to off.

system/systemd.go

@@ -19,6 +19,7 @@ const fakeMachineID = "42000000000000000000000000000042"
 
 type Unit struct {
 	Name    string
+	Enable  bool
 	Runtime bool
 	Content string
 	Command string