[placement] Add a bit about extraction plans to placement_dev
[placement] Add a bit about extraction plans to placement_dev

While we've not made any immediate plans to do the extraction we
should avoid adding complexity that would make it harder later, so
this new section discusses the plan to eventually extract, and the
structures that either help or hinder this.

Change-Id: Ia3e95e4b85aa768b8f94d4a99963c7ec719b8a13

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