Adding net-prerequisites.xml scenario - included in fast & smoke scenarios.

Adding net-setup.xml dependency on arping (sudo apt-get install iputils-arping)

Change-Id: I3276247dc8ae2a2c4ed47801385eb8e50be8f244
diff --git a/tools/test/scenarios/net-setup.xml b/tools/test/scenarios/net-setup.xml
index 7ad2ccc..702480e 100644
--- a/tools/test/scenarios/net-setup.xml
+++ b/tools/test/scenarios/net-setup.xml
@@ -34,8 +34,14 @@
         <step name="Wait-For-Mininet" requires="Start-Mininet"
               exec="onos-mininet wait 10"/>
 
-        <step name="Check-Summary" requires="Wait-For-Mininet"
-              exec="onos-check-summary ${OC1} [0-9]* 25 140 0"/>
+        <step name="Show-Network" requires="Wait-For-Mininet"
+              exec="onos-mininet sendAndExpect net --expect ."/>
+
+        <step name="Discover-Hosts" requires="Show-Network"
+              exec="onos-mininet sendAndExpect py [ h.cmd('arping -U -c 1 ' + h.IP()) for h in net.hosts ] --expect ."/>
+
+        <step name="Check-Summary" requires="Discover-Hosts"
+              exec="onos-check-summary ${OC1} [0-9]* 25 140 25"/>
 
         <step name="Balance-Masters" requires="~Check-Summary" if="${OC2}"
               exec="onos ${OC1} balance-masters"/>