mon: add osdmonitor_prepare_command to the admin socket

mon: add osdmonitor_prepare_command to the admin socket

It provides a developer path allowing functional tests to modify the
pending OSDMap without triggering a PaxosProposal.

It can be used as follows:

   echo '{"prefix":"osdmonitor_prepare_command","prepare":"osd crush tunables","profile":"bobtail"}' | nc -U out/mon.a.asok

It will transform the command into:

  {"prefix":"osd crush tunables","profile":"bobtail"}

and feed it to OSDMonitor::prepare_command_impl(). The pending OSDMap won't
be proposed because it short circuit PaxosService::dispatch. It will,
however, be proposed next time PaxosService::dispatch() gets a chance.

It cannot be used via the ceph command line.

Signed-off-by: Loic Dachary <loic@dachary.org>