After bulk create send DHCP notification

In the past send_dhcp_notification only handle single resource
because of VALID_RESOURCES = ['network', 'subnet', 'port'].
When I use bulk created ports to boot instance, the VMs could
not get IP address from DHCP agent. So I add some special handle
in function notify().

Add unit test for bulk create dhcp notification.

Modify a paramter for code style.

Change-Id: Ibf2b3380dc3081beaf0f2f68042b022f2726bbe7
Closes-Bug: #1247902
Merge "After bulk create send DHCP notification"