Do not create volume backed LB with ovn provider

(cherry picked from commit e3389c0)

Author: dosaboy

zaza/openstack/charm_tests/octavia/tests.py

@@ -500,6 +500,9 @@ def create_loadbalancer(self, ensure_volume_backed=False):
         else:
             vip_subnet_id = subnet_id
         for provider in self.get_lb_providers(self.octavia_client).keys():
+            if ensure_volume_backed and provider != 'amphora':
+                continue
+
             logging.info('Creating loadbalancer with provider {}'
                          .format(provider))
             final_exc = None