Fixes auto confirm invalid error

In the confirm_resize method at the compute API level, the 'metadata' attribute
is used to notify the instance usage and the 'system_metadata' attribute is
needed to get the old/new instance type values and the old vm_state, but the
auto confirm periodic task doesn't get the instance object with these two
attributes. This results in 'KeyError' exception being raised when the auto
confirm periodic task runs.
This commit sets expected_attrs to get 'metadata' and 'system_metadata' when
retrieving the instance object in the _poll_unconfirmed_resizes compute manager
method.

Reproduce steps in my devstack env with latest master branch nova:
1. set resize_confirm_window=10 in nova.conf
2. create an instance
3. resize it
4. wait for it is confirmed and watch the log

Fixes bug 1202975

Change-Id: I940f195d45fba4f2d25f9c517f11e99b74e97a8f
Merge "Fixes auto confirm invalid error"