Add AllowIsolate=yes to snapshots

TODO

@@ -52,8 +52,6 @@ Fedora 19:
 
 Features:
 
-* snapshots really should be isolatable, but currently aren't.
-
 * seems that when we follow symlinks to units we prefer the symlink
   destination path over /etc and /usr. We shouldn't do that. Instead
   /etc should always override /run+/usr and also any symlink

src/core/snapshot.c

@@ -256,6 +256,7 @@ int snapshot_create(Manager *m, const char *name, bool cleanup, DBusError *e, Sn
         }
 
         SNAPSHOT(u)->cleanup = cleanup;
+        u->allow_isolate = true;
         *_s = SNAPSHOT(u);
 
         return 0;