Add multi-worker support

Since https://review.openstack.org/#/c/37131/ got merged, Neutron
supports now multiple worker processes for API server.

Change-Id: Ib01240c39de21306892f4f92e0ba3ecb56837a7c
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit 64c452b4fb8b955639bc57b483f619224a23be83)
Backport to havana: https://review.openstack.org/#/c/60082/