Fix disable_network.sh and 'ip' path

Fix disable_network.sh and 'ip' path

Note: I propose to cherry-pick this change as currently there is no pod doing builts on master, so the only way is to check in stable/brahmaputra.

The 'ip' bin file can be located in different paths depending on the Linux
distribution so better not hardcode it to a specific path.

CentOS 6:     /sbin/ip
CentOS 7:     /usr/sbin/ip
Ubuntu 14.04: /sbin/ip

Change-Id: I0b02a221d3367ed4bfd8596992a2774f22138b54
Signed-off-by: Carlos Goncalves <carlos.goncalves@neclab.eu>
(cherry picked from commit a1bab6ecdfbfaf5521f31d9539e256fc6e0cac29)