Delete qg device during DVR-SNAT router deletion

Delete qg device during DVR-SNAT router deletion

In the DVR SNAT case, the 'qg' device was not deleted because
of patch:
https://review.openstack.org/#/c/151882/

During functional testing, the device is deleted
during the external bridge deletion. Because that happens after
the SNAT namespace is already deleted, it can cause a kernel
panic or ovs-vswitchd crash for certain OVS versions.

Also added assertions that all router interfaces were properly
cleaned up during functional testing, and enabled the unit tests
to catch this type of error.

Change-Id: Ica86a030860aa967d5b685b5bfb5546a85b2a809
Closes-Bug: #1418097