Change the Luxi operations structure

Currently, we define the LuxiOp type as a simple enumeration, and leave
the arguments structure to the users of the Ganeti.Luxi module. This is
suboptimal for a couple of reasons: first, we decouple the operation
type from operation arguments, and that means we don't use the type
system for validation of the arguments; second, the clients themselves
have to know about the JSON encoding of the protocol.

For the above arguments, we change the operation type to contain the
arguments too, and then the entire conversion/serialization is
restricted to the Ganeti.Luxi module. Also, the removal of the JSON
encoding from the clients results in an overall simplification of the
code.