Bump to Version 2.8.0 in process to release voltha-2.8

Change-Id: Idc8c34fc233329f60a4c543cfefd8db3b5047761
diff --git a/VERSION b/VERSION
index 9781df7..834f262 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.8.0-dev
+2.8.0