Forces flavorRef to be string in servers resize api

flavorid column is a string in the database model and
MySQL silently accepts either integer or string as
data type. However PostgreSQL expects the right data
type. So we force it to be string as early as possible
in the api.

Fixes Bug 1123525

Change-Id: Id117868b3a2f840f5d7ab463f3b79b6631b3b514
Merge "Forces flavorRef to be string in servers resize api"