Add list servers to large ops test

Add regression test for bug 1282842, which is hit when running nova list while
waiting for instances to move to ACTIVE.

Change-Id: Ic0714834846ad94b213d97e7756ca607e1f4d710
Related-Bug: #1282842
Make duration in serial test approximate total run time

The duration argument is supposed to mean how long the test will take to
finish. Before this change, if running serially it would take n*duration where
n is the number of test cases specified to run.

Change-Id: I9bd525b17e41fae811e3bf48642b728151e65737
Merge "Add list servers to large ops test"
Merge "Make duration in serial test approximate total run time"