Use (# of CPUs) osapi_volume_workers by default

Use (# of CPUs) osapi_volume_workers by default

This changes the default number of cinder API workers to be equal to the
number of CPUs available on the host, rather than defaulting to 1 as it
did before.

Commit 75c96a48fc7e5dfb59d8258142b01422f81b0253 did the same thing in
Nova in Icehouse. Similar changes are being made to Glance and Trove as
well.

DocImpact: osapi_volume_workers will now be equal to the number of
           CPUs available by default if not explicitly specified
           in cinder.conf.

UpgradeImpact: Anyone upgrading to this change that does not have
           osapi_volume_workers specified in cinder.conf will now be
           running multiple API workers by default when they restart
           the cinder-api service.

Closes-Bug: #1333370

Change-Id: I8dec104800c18618e0c8422bbb93d5dc19c12876