Deprecate conductor_workers in nova::api

Since we now have a dedicated nova::conductor class, the
conductor_workers parameter in nova::api isn't at his right place.
This commit deprecate the conductor_workers parameter in profit of
workers param in nova::conductor ( see https://review.openstack.org/106656/ )

Change-Id: I767b5c143f12a0f518828287b33aedbfd2201e72
Merge "Deprecate conductor_workers in nova::api"