Cleanup Elan instances when a network is deleted

Currently after creating some resources and deleting them, the ELAN instance(s) that were created in this process are not getting cleaned up.

Steps to reproduce:
1. In a fresh installation create a tenant neutron network, subnet and router.
2. Associate the subnet to the neutron router.
3. In karaf cli, check the ELAN instances.
opendaylight-user@root>elan:show
4. You would see an ELAN instance created for the resources.
5. Now dissociate the tenant subnet from neutron router, delete the router, network and subnet.
6. Check elan instances.
You will see the same output as in Step 3 above.

Expected behavior:
The ELAN instances should have been deleted.

Along with this, the following exception is logged in karaf.
2017-01-17 16:30:12,135 | ERROR | pool-43-thread-1 | InterfacemgrProvider | 335 - org.opendaylight.genius.interfacemanager-impl - 0.1.2.Boron-SR2 | Interface b1c4af31-b2eb-4cb7-8aff-f67fdbd1e8ac is not present
Exception in thread "pool-43-thread-1" java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:936)
at java.util.concurrent.ConcurrentHashMap.containsKey(ConcurrentHashMap.java:964)
at java.util.Collections$SetFromMap.contains(Collections.java:5459)
at org.opendaylight.netvirt.neutronvpn.NeutronvpnUtils.isNetworkTypeSupported(NeutronvpnUtils.java:975)
at org.opendaylight.netvirt.neutronvpn.NeutronNetworkChangeListener.remove(NeutronNetworkChangeListener.java:101)
at org.opendaylight.netvirt.neutronvpn.NeutronNetworkChangeListener.remove(NeutronNetworkChangeListener.java:34)
at org.opendaylight.genius.datastoreutils.AsyncDataTreeChangeListenerBase$DataTreeChangeHandler.run(AsyncDataTreeChangeListenerBase.java:129)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)