Fix aci_mod command. It should handle more complex operations now.
    
    The problem was trying to operate directly on the ACI itself. I
    introduced a new function, _aci_to_kw(), that converts an ACI
    into a set of keywords. We can take these keywords, like those passed
    in when an ACI is created, to merge in any changes and then re-create the
    ACI.
    
    I also switched the ACI tests to be declarative and added a lot more
    cases around the modify operation.