Get rid of create state in Autoscaling

Avoid storing the state of instances that are currently being created in
the instance group resource itself. Instead, ensure that they are returned
from handle_create() and take advantage of that result being passed back in
to check_active().

Change-Id: Ie2c75eaee620aa0599665168bfb92b345128d392
Clean up metadata refresh unit test

The stack was not being fully created, because the calls to the mock
functions were not present for the second instance.

Change-Id: I894473533eb40133e4c22facbbee749a971cec89
Merge "Get rid of create state in Autoscaling"
Merge "Clean up metadata refresh unit test"