In Ubuntu 20.04.1 LTS, a path of iptables is not /sbin/iptables but
/usr/sbin/iptables. So, current code gets an error with
"/sbin/iptables" failed : No such file or directory
This commit fixes to use iptables when neutron L3 service is enabled on
Ubuntu 20.04.1 LTS.
Change-Id: I76eb89a2ae26431065cd19f0af235e71eb9f4169