Remove all l3 related namespaces in case of multiple active routers in
l3 high available mode. The root cause is that keepalived does not
remove the vip address from nic during starting.
neutron-vpnaas-agent is subclass of l3 agent, so should remove all l3
related namespace before starting vpnaas agent.
Closes-Bug: #1703078
Depends-On: Ic9417d2eb03e0dd93f7c668b189b4ad9c72eae0f
Change-Id: I05c1faf2551bb5e70c299e884adf58cd2af52739
58964d6825