Reducing duplicated values on values files for SiaB

Reduced the amount of duplicated values between seba-ponsim values file.
To deploy, for example, the latest of seba-ponsim, now both seba-ponsim.yaml and seba-pomsim-latest.yaml values files need to be used with the specified order.
Makefile for SiaB is modified accordingly in the patch https://gerrit.opencord.org/#/c/16743/.
Also:
- removed stratumEnabled value from Mininet chart and added mnStratumSwitchCount
- moved bngAppUrl and bngAppVersion values on root of values file

Change-Id: I26b8e91f868646adc06f2c1feea72b333defd487
diff --git a/mininet/values.yaml b/mininet/values.yaml
index 1916755..8437a3f 100644
--- a/mininet/values.yaml
+++ b/mininet/values.yaml
@@ -49,7 +49,7 @@
             - olt
         topologyKey: kubernetes.io/hostname
 
-stratumEnabled: false
+mnStratumSwitchCount: 0
 topoScript: '/toposcripts/topo.py'
 onosOpenflowSvc: "onos-openflow.default.svc.cluster.local"
 vlanMatchDepth: 2