Use short form for references

Since documentation is now derived from the validation framework, it makes sense that our internal representation should match the user's ideal representation. When a command needs an attribute to reference an existing object, the code previously used a long form (`{"name": "value"}`) rather than the shortcut form (`"value"`). This corrects the internal representation without affecting the external.

One side effect of this change is that `create-policy` will no longer create tags as a side effect.