Use stevedore's make_test_instance

Replace the use of TestManager with make_test_instance
to get an instance of the type of manager expected.
The old TestManger class is deprecated.

Change-Id: I73f3202ac7a1420aaa303d5ea2e741b2fe5673e9
Partial-bug: #1273455
Merge "Use stevedore's make_test_instance"