Don't setup identity v3 client if v3 is disabled

Some tests report failures about the identity v3 url not being
specified, even if api_v3 is set to false in tempest.conf.

Change-Id: Ifb5c6c8bd154373d33c90236ff37fcc5b8759405
Closes-Bug: #1389742
diff --git a/tempest/clients.py b/tempest/clients.py
index 19b4e11..438d386 100644
--- a/tempest/clients.py
+++ b/tempest/clients.py
@@ -291,7 +291,8 @@
                 self.telemetry_client = TelemetryClientXML(
                     self.auth_provider)
             self.token_client = TokenClientXML()
-            self.token_v3_client = V3TokenClientXML()
+            if CONF.identity_feature_enabled.api_v3:
+                self.token_v3_client = V3TokenClientXML()
             self.volume_availability_zone_client = \
                 VolumeAvailabilityZoneClientXML(self.auth_provider)
             self.volume_v2_availability_zone_client = \
@@ -397,7 +398,8 @@
                 self.telemetry_client = TelemetryClientJSON(
                     self.auth_provider)
             self.token_client = TokenClientJSON()
-            self.token_v3_client = V3TokenClientJSON()
+            if CONF.identity_feature_enabled.api_v3:
+                self.token_v3_client = V3TokenClientJSON()
             self.negative_client = rest_client.NegativeRestClient(
                 self.auth_provider)
             self.negative_client.service = service