Use gateway only for 'public' net in bonding test

In system tests for bonding feature we use IP subnet
for public network (/24) for other networks which are
assigned to bonded interface by splitting it on /27
subnets. But we can't use public network router as
gateway for them.

Change-Id: I0291f227cf84f1cdce11c801a044d51f24448654
Closes-bug: #1403007
Merge "Use gateway only for 'public' net in bonding test" into stable/6.0