This PS attempts to make the Nova bootstrap job a little speedier.
It's been noticed that flavor check/creation on initial deployment
are rather slow, so this backgrounds the creation of each flavor,
so that the defined flavors can be checked/create in parallel,
rather than one at a time. Waits for the jobs to finish at the end.
Change-Id: Ib9ab345e5aee697a41414e927910335dd286072f
9a8a476d9f