Expose sizelimit option to config generator

* Adding a group name to prevent collision with keystone
  middleware library option
* Adds an opts module for the config generator to use.
* Makes the option in sizelimit private since we don't want consumers
  using them directly.
* Moves the options to an oslo_middleware group with appropriate
  deprecated_opts settings to keep existing configs working.

Closes-Bug: #1368490
Change-Id: I72263bd363a79275a314de727a04277276866565
Merge "Expose sizelimit option to config generator"