Add boolean convertor to "create multiple servers" API

"create multiple servers" API contains "return_reservation_id" as an
attribute, and the attribute should be boolean. There are the other
boolean attributes such as "on_shared_storage" of evacuate API, they
can be passed with either boolean or string(e.g. "True"). For the
consistency of whole Nova APIs, this patch adds boolean convertors to
multiple_create API.

Change-Id: Id6f458a2ced869a31d5d25d661309973132e9360
Closes-Bug: #1281440
Merge "Add boolean convertor to "create multiple servers" API"