When determining placements for a related set of resources
via OpenStack Heat, there's no a priori knowledge of specific
location candidates. Thus the /v1/plans API never accounted
for them. However, when it comes to ad hoc placement of a
single resource (e.g., via OpenStack Nova), such knowledge
*does* exist. This is one piece of a three-part change (same
issue tracking ID).
Change-Id: I327b87a1a1104019239547d03600cdd849ea6501
