Modify ipset functional tests to pass on older machines

Modify ipset functional tests to pass on older machines

Production code uses ipset exclusively in the root namespace,
however functional testing uses ipset in namespace for isolation.
This poses an issue as ipset is not supported in namespaces on
all kernels and distributions (I'm looking at you CentOS/RHEL 7.1).

This patch changes the ipset functional tests to work in the root
namespace while taking care of cleanups.

Conflicts:
	neutron/tests/functional/agent/linux/test_ipset.py

Change-Id: I08b2f59197ed76e59b2e58b5a10820653e857cda
Closes-Bug: #1460220
(cherry picked from commit 96e2314c8c0f06967b53828b0a771973f0031243)