htools: start reorganising the IAllocator workflow

htools: start reorganising the IAllocator workflow

Currently, all IAllocator requests return the same result type: a list
of strings (usually nodes, but can also be instance:nodes). This won't
be true in the future, with the new request types that will return
opcode lists.

Therefore we will need to support multiple result types in the
workflow; the first step is to make the final formatResponse step
result-type-indepedent, so we take out the formatRVal invocation out
of this function.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>