Replace nova.utils.cpu_count() with processutils.get_worker_count()

cpu_count() was moved from nova to oslo-incubator via commit
85f178489a128a04a7ee3ed44018403caa109ef0 so it can be used in other
projects.

This change removes the duplicate code from nova and uses the renamed
method from oslo.

Change-Id: I03da77adf64e0495880343b7086f5907a2c5f75d
Merge "Replace nova.utils.cpu_count() with processutils.get_worker_count()"