before this change, we always send the parameters with query strings,
but we will add an API ("storage_service/restore") which accepts its
parameters in HTTP body as well.
in this change, we add an optional parameter to `do_request()` and
`post()`, so that we can send HTTP body when using "POST" method
in nodetool implementation.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>