[tests] Don't fail if kill cmd says that process don't exists
[tests] Don't fail if kill cmd says that process don't exists

In e.g. functional tests, if kill command called by
RootHelperProcess.kill() method will return error that process
with specified PID don't exists, test should not fails.

This patch adds handling of such case in this method and reraise
exception always if error code is different than 1 or
raised exception have got different error message.

Change-Id: I92c8f74f1dd2e76141e1e024a22589e9ddc4ff57
Closes-Bug: #1843418

openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron