Add /usr/sbin and /sbin to $PATH route

Depending on the system configuration, /usr/sbin and /sbin can be not
available to the user executing the script, and failing to boot
the seed image due to the missing route to the arp command.

In Fedora, Ubuntu and Debian arp is located in /usr/sbin, meanwhile
in Centos it's located in /sbin .

Change-Id: I34be00348626205929329962d7e2998b4dca55f3