Unify checks for TF backend

Method of checking already presented in the tempest.
We can only reuse it.

Related-Prod: PRODX-37464
Change-Id: I5e75582081843a0530094c4f2ee0ce32360b31c0
(cherry picked from commit 7fcab05728f4cefdb2691df983444d2c204c0eeb)
diff --git a/octavia_tempest_plugin/config.py b/octavia_tempest_plugin/config.py
index 0587a5c..58ddf13 100644
--- a/octavia_tempest_plugin/config.py
+++ b/octavia_tempest_plugin/config.py
@@ -15,7 +15,6 @@
 
 from oslo_config import cfg
 from oslo_log import log as logging
-from tempest import config
 
 from octavia_tempest_plugin.common import constants as const
 
@@ -311,12 +310,3 @@
                      'scope? This configuration value should be same as '
                      'octavia.conf: [oslo_policy].enforce_scope option.'),
 ]
-
-
-def is_tungstenfabric_backend_enabled():
-    """Return True if TungstenFabric is used as a backend."""
-    try:
-        sdn = getattr(config.CONF, 'sdn')
-        return getattr(sdn, 'service_name', None) == 'tungstenfabric'
-    except cfg.NoSuchOptError:
-        return False
diff --git a/octavia_tempest_plugin/tests/test_base.py b/octavia_tempest_plugin/tests/test_base.py
index e758be3..55e4d1b 100644
--- a/octavia_tempest_plugin/tests/test_base.py
+++ b/octavia_tempest_plugin/tests/test_base.py
@@ -34,7 +34,6 @@
 
 from octavia_tempest_plugin.common import cert_utils
 from octavia_tempest_plugin.common import constants as const
-from octavia_tempest_plugin import config as config_octavia
 from octavia_tempest_plugin.tests import RBAC_tests
 from octavia_tempest_plugin.tests import validators
 from octavia_tempest_plugin.tests import waiters
@@ -875,7 +874,7 @@
             cls.webserver2_public_ip))
 
         if (CONF.load_balancer.test_with_ipv6 and not
-                config_octavia.is_tungstenfabric_backend_enabled()):
+                config.is_tungstenfabric_backend_enabled()):
             # Enable the IPv6 nic in webserver 1
             cls._enable_ipv6_nic_webserver(
                 cls.webserver1_public_ip, cls.lb_member_keypair['private_key'],
@@ -1055,7 +1054,7 @@
                 network_id=network['id'],
                 mac_address=instance_network[0]['OS-EXT-IPS-MAC:mac_addr'])
             port_id = result['ports'][0]['id']
-            if config_octavia.is_tungstenfabric_backend_enabled():
+            if config.is_tungstenfabric_backend_enabled():
                 port = result['ports'][0]
                 fixed_ip = None
                 for ip in port["fixed_ips"]: