Preemptible instances run on a configurable host aggregate, using the
freepool by default.
Compared to the proposed spec, the use of a separate aggregate allows to
better control scheduling of preemptibles. For example, we can prevent
preemptible instances from being launched on hosts that are about to be
used by reservations. To be used, this feature will need additional
support in the main Blazar service.
Blueprint: blazar-preemptible-instances
Change-Id: I294c22b22a1d9e3df01c56a518476a33782d1196
3d01888c14