Enable test_create_multiple_servers test for V3 API

Now that the multiple create extension has merged we can enable
this test for the V3 API. Need to also override post_server
for the test client for the V3 API as the data return when requesting
a reservation id in the V3 API has changed

Change-Id: I7f34f1e90b872c4679b5c54fe9e4b0f4420910ab
Closes-Bug: #1206884
Merge "Enable test_create_multiple_servers test for V3 API"