Check the kernel feature

Check the kernel feature

From: Daniel Lezcano <daniel.lezcano@free.fr>

The virtual devices are automatically destroyed when the network namespace
dies for the kernel version >= 2.6.29. Until this version the network devices
have to be destroyed by lxc. This modification checks the version of the
kernel to make lxc to destroy the network devices or not.

Signed-off-by: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>