Cells: Commit resize quota reservations immediately

There's not an immediate good solution for quotas on resize with cells
due to how it's implemented.  Will have to revisit this with a better
patch later.  This can lead to some out-of-sync issues when deleting
things in certain resize states... or if there are resize errors.

Store migration information on API cell to correctly handle quotas

Revert and confirm of migrations require access to migration information
so they can calculate the reservation correctly. For new resizes, store
a migration record locally so it can be used later on. For old resizes,
keep the previous behavior (for now).

Implements blueprint nova-compute-cells

Change-Id: I356600d1d0d9408b019a9422f795b4244c252052
Merge "Cells: Commit resize quota reservations immediately"