Correct 'list index out of range' error in dhcp agent test

test_list_networks_hosted_by_one_dhcp will raise
'list index out of range' exception when no dhcp agent present.
assertNotEmpty should be used instead of assertIsNotNone to
check the dhcp agent's presence

Change-Id: I1af72a5a1ba36781fa97ff5f40ee0f229ef3d330
Closes-Bug: #1632557
1 file changed