devstack: enable placement-api
devstack: enable placement-api

n-cpu requires placement-api section in nova.conf.
Without it, n-cpu aborts as follows.
Enable placement-api so that configure_placement_nova_compute is called
to configure placement section in nova.conf.

> ERROR oslo_service.service [-] Error starting thread.
> ERROR oslo_service.service Traceback (most recent call last):
> ERROR oslo_service.service   File "/usr/local/lib/python2.7/dist-packages/oslo_service/service.py", line 722, in run_service
> ERROR oslo_service.service     service.start()
> ERROR oslo_service.service   File "/opt/stack/new/nova/nova/service.py", line 144, in start
> ERROR oslo_service.service     self.manager.init_host()
> ERROR oslo_service.service   File "/opt/stack/new/nova/nova/compute/manager.py", line 1136, in init_host
> ERROR oslo_service.service     raise exception.PlacementNotConfigured()
> ERROR oslo_service.service PlacementNotConfigured: This compute is not configured to talk to the placement service. Configure the [placement] section of nova.conf and restart the service.
> ERROR oslo_service.service
> acement-api.

Change-Id: I931f1e46de0d65dfecb3e34a2bb2bfab561c1164
Closes-bug: #1660895

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl