CLOUDSTACK-4442: Include a test for creating networks with sourcenat only

The test attempts to create a network with offering serving only
dhcp,dns and sourcenat. However the source NAT functionality itself
isn't tested as reported in the bug. So the test will pass.

TODO: come up with a way to test source nat without enabling additional
services for pf/staticnat

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
CLOUDSTACK-4437: Fix iso usage event count to match the number of image stores

Signed-off-by: Prasanna Santhanam <tsp@apache.org>