Fix up instance types in sys meta for resizes

This adds code to the resize paths to update the system_metadata
with new (or reverted) flavor information so that resized instances
are consistent with normal ones.

It also stashes the new and old instance_type information in system_metadata
during resize, which allows us to avoid database lookups for type
information at finish, confirm, and revert time.

Related to blueprint no-db-compute

Change-Id: I7f74ef8131be7a60e3a844fdf00fdeb3683f1317
Merge "Fix up instance types in sys meta for resizes"