Don't copy the flavorid when updating flavors

The Nova API makes the implicit assumption that flavorids are unique,
so if one creates two flavors with the same flavorid, subsequent
requests for flavor information can vary between "inaccurate" and
"wildly f*cking inaccurate"

From an end-user perspective, it makes it look like data such as
flavor access settings get lost between edits (because the API pulls
the access list from the wrong flavor)

Let the API assign a new flavorid on changes.  The comments around
the handler code imply that is the desired behaviour, anyhow.

Cherry-picked from 4100a1cbc24184b58d5049dfb601b18e29e6107d

Change-Id: I5403202396a7b136b0ae8637d6aae1cf32550747
Closes-Bug: 1276371