Use get_tenant_network in get_server_ip

This is to use get_tenant_network to get ssh
network when CONF.validation.network_for_ssh is
not specified. Using get_tenant_network will be
more suitable here because when net_creds do not
have valid network attribute, it will look up
network according to CONF.compute.fixed_network_name.

Change-Id: Ifae1f64b74296b09d416c6f212d9914e25d8bec9
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index 867c0fc..e5d5c69 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -675,9 +675,7 @@
                 addresses = server['addresses'][
                     CONF.validation.network_for_ssh]
             else:
-                creds_provider = self._get_credentials_provider()
-                net_creds = creds_provider.get_primary_creds()
-                network = getattr(net_creds, 'network', None)
+                network = self.get_tenant_network()
                 addresses = (server['addresses'][network['name']]
                              if network else [])
             for address in addresses: