Fix the exception for a nonexistent flavor

flavor_get_by_flavor_id() raises a FlavorNotFound exception when trying to
delete a nonexistent flavor and REST API layer should catch the exception.
However, one of v2 API catches a NotFound exception instead, and it has
been fixed on v3 API.
This patch fixes the one of v2 API.

This patch is required for v2.1 API development. On the development, we are
sharing current v2 unit tests between v2 and v2.1 for avoiding duplicated
code. Now on v3 API which is source of v2.1, "delete a flavor" API catches
a FlavorNotFound exception. So current unit test test_delete fails against
v2.1 API if not merging this patch, because current v2 unit test raises more
generic exception NotFound but v3 "delete a flavor" API catches more specific
exception FlavorNotFound.

Partially implements blueprint v2-on-v3-api

Change-Id: Iec7bca55f0e9f4b8a85f3b9cd54d8deff2904f86
Merge "Fix the exception for a nonexistent flavor"