Enable multi-process for API service

Enable multi-process for API service

Due to the limit of Python interpreter, API service of Cinder can't
really utilize underlying multi-core architecture even libraries
like eventlet has been used. To make API service much more scalable,
we'd adopt multi-process (worker) mode that has been used for long
in Glance/Swift/Nova.

The default behavior isn't changed with this patch, Cinder API
service will still run in one process (default value of
osapi_volume_workers is None).

Implementation wise, a good portion of cinder/service.py has been
removed because those content has been merged in Oslo version of
service module.  cinder/wsgi.py is also updated to adopt the change
for multiple WSGI servers running in separate processes.

Implement bp: multi-process-api-service

DocImpact: 'New config option osapi_volume_workers is used to specify
number of API service workers (OS processes) to launch for Cinder
API service.  Setting this config option to a proper value (e.g.
osapi_volume_workers = # of CPU cores/threads of the machine) can
greatly improve the total throughput of API service [# of API
requests can be handled per second].'

Also removed out-dated comments in bin/cinder-api due to the fact
that this bug [1] has been fixed in eventlet 0.9.13

[1] https://bitbucket.org/eventlet/eventlet/issue/92/eventletgreen-override-of-oswaitpid

Change-Id: I8361d0dc0d43040e48634ff1aee1324e5e0af466