Use instance data instead of flavor in simple_tenant_usage extension

The simple_tenant_usage extension gets the flavor data from the instance
and then looks up the flavor from the database to return usage information.
Since we now store all of the flavor data in the instance itself,
we should use that information instead of what the flavor currently
says is right. This both (a) makes it more accurate and (b)
avoids us failing to return usage info if a flavor disappears.

Change-Id: I1472d78e8e920013a1db5c76e04715fa04b696c2
Closes-Bug: #1276731
Merge "Use instance data instead of flavor in simple_tenant_usage extension"