[placement] Update placement devref to modern features
[placement] Update placement devref to modern features

Adjust placement.rst so that it mentions traits and links to more
specs (rather than pending reviews).

Change-Id: Ic4e3ed8ddf9b2b4eb91c56d53ec72335ad3919c9

gooddata/openstack-nova,openstack/nova,vmturbo/nova,mahak/nova,Juniper/nova,vmturbo/nova,rahulunair/nova,klmitch/nova,Juniper/nova,klmitch/nova,Juniper/nova,jianghuaw/nova,mikalstill/nova,openstack/nova,phenoxim/nova,vmturbo/nova,rahulunair/nova,jianghuaw/nova,mahak/nova,mikalstill/nova,jianghuaw/nova,Juniper/nova,klmitch/nova,jianghuaw/nova,vmturbo/nova,mikalstill/nova,klmitch/nova,phenoxim/nova,gooddata/openstack-nova,gooddata/openstack-nova,mahak/nova,rahulunair/nova,gooddata/openstack-nova,openstack/nova