Create utility to clean-up netns.

Create utility to clean-up netns.

Fixes bug 1035366

Adds namespace clean up utility called quantum-netns-cleanup which can be used
to remove old namespaces.

The --force option can be used to remove all Quantum namespaces and any
remaining devices.  The force option is should not be run on a live Quantum
systems.  It is intended for cleaning up devstack a after running
unstack.sh (ideally this will be added to unstack.sh in the future).

Example cmd line when cleaning up a devstack install:
quantum-netns-cleanup --config-file /etc/quantum/quantum.conf \
--config-file /etc/quantum/dhcp_agent.ini --force

Change-Id: I6cf153df21e83bff2cde816db12b22102d1ba698