Make a flavorRef validation strict

V2.0 API doesn't allow empty string as flavorRef and it returns a
BadRequest response if receiving it.
Now V2.1 API doesn't check it now and this patch makes V2.1 API check.
In addition, this patch adds tests for checking both API behaviors.

Partially implements blueprint v2-on-v3-api

Change-Id: I6c80b66102f00c832922cb507b82e9f3b534557f
Merge "Make a flavorRef validation strict"