Fix problem of deleting dhcp port

when network is deleted, the dhcp port in that network will be deleted
automaticly, so '_cleanup_ports' should not clean dhcp port. I already
have a patch to fix the problem of deleting dhcp port.

Change-Id: I92f90a3dad6d76954d466e4b30ab7c46434ba7df
Closes-bug: #1279683
diff --git a/tempest/common/isolated_creds.py b/tempest/common/isolated_creds.py
index ac8b14f..c54a8e8 100644
--- a/tempest/common/isolated_creds.py
+++ b/tempest/common/isolated_creds.py
@@ -456,7 +456,8 @@
             port
             for port in self.ports
             if (port['network_id'] == network_id and
-                port['device_owner'] != 'network:router_interface')
+                port['device_owner'] != 'network:router_interface' and
+                port['device_owner'] != 'network:dhcp')
         ]
         for port in ports_to_delete:
             try: