Creates multiple worker processes for API server

This change to the WSGI code uses openstack.common.service to create
multiple worker processes to handle API load.  The main process will
start up a configurable (workers=??) number of child processes which
will all listen on the bind port.  The main process becomes the parent
and manages the children.  The parent is not a worker.

Backwards compatibility is preserved by setting api_workers to 0, the
default.  In this case, no separate worker processes are spawned and
the worker threads run in the main process.

Implement blueprint multi-workers-for-api-server

Change-Id: Iffa76041d0055840ccca852814b0e71f17a950ac