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
Add exception handling in "nova diagnostics"

when using "nova diagnostics" with a not running
instance, user will get an incorrect message. This
change handles that exception.

Change-Id: Ia2aa76a5c2c2aeed446411700d74d547dd73cf90
Closes-bug: #1289079
Merge "Add exception handling in "nova diagnostics""
Merge "Use instance data instead of flavor in simple_tenant_usage extension"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch