Use admin context when removing DVR router on vm port deletion

Use admin context when removing DVR router on vm port deletion

In case non-admin tenant removes last VM on a shared network (owned
by admin) connected to a DVR router (also owned by admin) we need
to remove the router from the host where there are no more dvr
serviceable ports. Commit edbade486102a219810137d1c6b916e87475d477
fixed logic that determines routers that should be removed from host.
However in order to actually remove the router we also need admin
context.

This was not caught by unit tests and one reason for that is so called
'mock everything' approach which is evil and generally useless.
This patch replaces unit tests with functional tests that we able
to catch the bug.

Closes-Bug: #1424096
Change-Id: Ia6cdf2294562c2a2727350c78eeab155097e0c33