Moving voltha-2.5 to use voltha-onos 4.1.8

Change-Id: Ia44ef3cbc6dc891e55d2fcaff6c6d58d9a6bd081
diff --git a/releases/voltha-2.5 b/releases/voltha-2.5
index 81e3954..a35a940 100644
--- a/releases/voltha-2.5
+++ b/releases/voltha-2.5
@@ -18,4 +18,4 @@
 # kind-voltha's default which is 'master'.
 # Also we need to specify the tag for voltha/voltha-onos image since this is
 # not in the ONOS chart.
-export EXTRA_HELM_FLAGS="--set defaults.image_tag=null,images.onos.tag=4.1.7 "
+export EXTRA_HELM_FLAGS="--set defaults.image_tag=null,images.onos.tag=4.1.8 "