[QoS] Clear rate limits when default null values are used
[QoS] Clear rate limits when default null values are used

In that case before, tc was called with zero rate and burst values.
Newer iproute versions (>= 4.9) do not allow that anymore, so we send a
clear command instead

This also fixes QoS L3 agent extension functional tests run with such an
iproute version (CentOS 7.5, Ubuntu 18.04)

Change-Id: Idf16e2460bd434f5eab79745558c55c6579e81e8
Closes-Bug: #1782141

mahak/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,huntxu/neutron,noironetworks/neutron,openstack/neutron,huntxu/neutron,openstack/neutron,mahak/neutron