Fix bug in db.instance_type_destroy

Fix db.instance_type_destroy()
Corresponding instance_type_projects weren't deleted

Fix db.instance_type_acces_get_by_flavor_id method
Return [] list if there is no such instance_type
instead of AttributeError

Fixes bug 1172153

Change-Id: Ib1a9a23aa6e26711c198bf4a5059797044c44426
Merge "Fix bug in db.instance_type_destroy"