iallocator.rst: refactor for readability; minor improvements

This commits breaks down the "Input message" section of iallocator.rst into
two separate subsections: one detailing keys that are required in all
operation types; a second one detailing the "request" element, which is
different for each type of request.

Some other minor improvements are included as well:

  - update input example to version 2, and add the "nodegroups" and
    "enabled_hypervisors" top-level elements, and the "group" and
    "hypervisor" attributes for nodes and allocation request, respectively.

  - sort keys in the example dictionaries according to the order in earlier
    sections, for easy comparison of documentation with its examples.

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>