Fixing stc prerequisites to allow ONOS_USE_SSH to be either true or unset/blank.

Change-Id: I27604f28c34ff7a451847a6b3772d300b46d3d57

@@ -15,9 +15,11 @@
   -->
 <scenario name="prerequisites" description="ONOS test pre-requisites">
     <group name="Prerequisites">
-        <step name="Check-Environment" exec="test -n ${ONOS_ROOT} -a -n ${ONOS_NIC} -a -n ${OC1} -a '${ONOS_USE_SSH}' == 'true'"/>
+        <step name="Check-Environment"
+              exec="test -n ${ONOS_ROOT} -a -n ${ONOS_NIC} -a -n ${OC1}"/>
+        <step name="Check-Secure-SSH"
+              exec="test '${ONOS_USE_SSH}' == 'true' -o '${ONOS_USE_SSH}' == ''"/>
         <step name="Check-ONOS-Bits" exec="onos-check-bits"/>
-
         <parallel var="${OC#}">
             <step name="Check-Passwordless-Login-${#}"
                   exec="ssh -n -o ConnectTimeout=3 -o PasswordAuthentication=no ${ONOS_USER}@${OC#} date"/>