Prevent used flavor from being deleted

When a flavor deletion is attempted, two checks are necessary:
* ensure the flavor is not deployed on an instance
* ensure the flavor is not associated with a deployment role

This patch adds the second check which was missing.

Change-Id: I8c9610a4245b617cae281b44084d1a93b4699c2c
Closes-bug: #1302019
Merge "Prevent used flavor from being deleted"