iallocator.rst: refactor for readability; minor improvements
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 <16293852a0900193fe70fa7d24f714f37ba99a56@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

bitemyapp/ganeti,dimara/ganeti,ganeti/ganeti,yiannist/ganeti,ganeti/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,andir/ganeti,onponomarev/ganeti,apyrgio/ganeti,andir/ganeti,apyrgio/snf-ganeti,apyrgio/snf-ganeti,ganeti/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,andir/ganeti,mbakke/ganeti,grnet/snf-ganeti,leshchevds/ganeti,dimara/ganeti,bitemyapp/ganeti,yiannist/ganeti,mbakke/ganeti,grnet/snf-ganeti,onponomarev/ganeti,mbakke/ganeti