Fix some startup issues now that we use cluster size

- Change max cluster size to size of largest cluster
- Cleanup some reading of params
- Catch when ONOS isn't fully started

Change-Id: I5b2ca791046bd4061d31355276958ead39f8b65c
(cherry picked from commit ace1f317e19fd50924e78e53b566d69db23f5683)
diff --git a/TestON/tests/USECASE/SegmentRouting/SRBridging/SRBridging.topo.physical b/TestON/tests/USECASE/SegmentRouting/SRBridging/SRBridging.topo.physical
index 37e1830..36842e0 100644
--- a/TestON/tests/USECASE/SegmentRouting/SRBridging/SRBridging.topo.physical
+++ b/TestON/tests/USECASE/SegmentRouting/SRBridging/SRBridging.topo.physical
@@ -19,7 +19,7 @@
                 <rest_port></rest_port>
                 <prompt></prompt>  # TODO: we technically need a few of these, one per component
                 <onos_home></onos_home>  # defines where onos home is on the target cell machine. Defaults to entry in "home" if empty.
-                <nodes>3</nodes>  # number of nodes in the cluster
+                <nodes> 7 </nodes>  # number of nodes in the cluster
             </COMPONENTS>
         </ONOScell>