Functional tests for keepalived should spawn processes in namespaces
where dummy interfaces used in keepalived.conf file exists.
Otherwise keepalived 2.0.10 (this is version used currently in RHEL 8)
fails to start and tests are failing.
On older versions of keepalived, like 1.3.9 used in Ubuntu 18.04,
keepalived is logging warning about not existing interfaces but it's
starting fine thus tests are running properly.
So this patch adds creation of namespace for each test from
neutron.tests.functional.agent.linux.test_keepalived module,
creates dummy interfaces with names used in keepalived config file
and runs keepalive process in this namespace.
Change-Id: I54f45b8c52fc1ecce811b028f0f92e0d78d3157b
Closes-Bug: #1830232
959af761cb