Allow to create a flavor without specifying id

On current v2 API, "create a flavor" API allows a request without
specifying "id" parameter. The API of v3 API, which is source of
v2.1, doesn't allow it now. However, it will cause a backwards
incompatible issue when implementing v2.1 API.
This patch makes v3 API allow it.

Partially implements blueprint v2-on-v3-api

Change-Id: I2d3fda323df57f8c0c116347e3c4a38c807de31b
Merge "Allow to create a flavor without specifying id"