command:serve_commands: allow unspecified parameters (use defaults).

command:serve_commands: allow unspecified parameters (use defaults).

Also raise UnknownCommand if there is no `command` key in the posted
dict (malformed request).

With the new code, you can run commands with:

  $ wget --post-data='command: list' http://localhost:8000/run/

instead of having to go through and specify all the parameters
explicitly.  This will make the command server more robust for use
with older clients (who may not know about all the parameters that the
server knows about).  Parameters sent by the client that the server
does not know about are silently ignored.