Forced isolation for the tests in test_list_servers_negative.py

In bug 1252971, race condition will arise if the allow_tenant_isolation
is false. ListServersNegativeTestJSON and ListServersNegativeTestXML
will share the tenant, if both of them check if there are existing servers
or not before both of them create new servers, the actual servers will
be double of the expected.
this also removes the redundant code in this files.

Closes-bug: 1252971

Change-Id: I2f859057a71ccada86b5733c1a27386fcd6bc4b5

tempest/api/compute/servers/test_list_servers_negative.py

@@ -17,71 +17,19 @@
 
 import datetime
 
-from tempest.api import compute
 from tempest.api.compute import base
-from tempest import clients
 from tempest import exceptions
 from tempest.test import attr
 
 
 class ListServersNegativeTestJSON(base.BaseV2ComputeTest):
     _interface = 'json'
-
-    @classmethod
-    def _ensure_no_servers(cls, servers, username, tenant_name):
-        """
-        If there are servers and there is tenant isolation then a
-        skipException is raised to skip the test since it requires no servers
-        to already exist for the given user/tenant.
-        If there are servers and there is not tenant isolation then the test
-        blocks while the servers are being deleted.
-        """
-        if len(servers):
-            if not cls.config.compute.allow_tenant_isolation:
-                for srv in servers:
-                    cls.client.wait_for_server_termination(srv['id'],
-                                                           ignore_error=True)
-            else:
-                msg = ("User/tenant %(u)s/%(t)s already have "
-                       "existing server instances. Skipping test." %
-                       {'u': username, 't': tenant_name})
-                raise cls.skipException(msg)
+    force_tenant_isolation = True
 
     @classmethod
     def setUpClass(cls):
         super(ListServersNegativeTestJSON, cls).setUpClass()
         cls.client = cls.servers_client
-        cls.servers = []
-
-        if compute.MULTI_USER:
-            if cls.config.compute.allow_tenant_isolation:
-                creds = cls.isolated_creds.get_alt_creds()
-                username, tenant_name, password = creds
-                cls.alt_manager = clients.Manager(username=username,
-                                                  password=password,
-                                                  tenant_name=tenant_name)
-            else:
-                # Use the alt_XXX credentials in the config file
-                cls.alt_manager = clients.AltManager()
-            cls.alt_client = cls.alt_manager.servers_client
-
-        # Under circumstances when there is not a tenant/user
-        # created for the test case, the test case checks
-        # to see if there are existing servers for the
-        # either the normal user/tenant or the alt user/tenant
-        # and if so, the whole test is skipped. We do this
-        # because we assume a baseline of no servers at the
-        # start of the test instead of destroying any existing
-        # servers.
-        resp, body = cls.client.list_servers()
-        cls._ensure_no_servers(body['servers'],
-                               cls.os.username,
-                               cls.os.tenant_name)
-
-        resp, body = cls.alt_client.list_servers()
-        cls._ensure_no_servers(body['servers'],
-                               cls.alt_manager.username,
-                               cls.alt_manager.tenant_name)
 
         # The following servers are created for use
         # by the test methods in this class. These