Cleanup existing instances in setup for test_list_servers_negative.py

Change I8519cba1fcf6ff39d69839783d07359544509db5 was supposed to make
the setup for test_list_servers_negative.py handle the case that tempest
is being ran without tenant isolation, but it doesn't fix the entire
problem because it's only checking if the MULTI_USER global is False.
The MULTI_USER global is only False if allow_tenant_isolation=False and
user1 == user2. So if you're running tempest without tenant isolation
but you do have two separate users/tenants for running the tests, then
the setup can still fail.

This patch fixes the problem by always doing the cleanup if
allow_tenant_isolation=False regardless of what MULTI_USER is set to.

Closes-Bug: #1234370

Change-Id: I821e200d07c11ba04d8be788095b7648de1d820b
add volume list tests

list volume using parameters:
display_name
status
availability_zone

Change-Id: I97d88b5033e7033cfc4b2328698978da7714f0a4
Merge "add volume list tests"
Merge "Cleanup existing instances in setup for test_list_servers_negative.py"