[AETHER-443][AETHER-453] Improve tost-onos image builder

- The workspace is built from scratch (onos and apps)
- Apps are built using local version or pulled from mvn repos
- 'tost-onos' is a minimal onos image built using 'tost' profile
- 'tost' is an onos image buit on top of 'tost-onos' with the tost apps

Change-Id: Id8ac664d16e6eaade658ba5bc41de1359aae09c5
diff --git a/ONOS_VERSION b/ONOS_VERSION
new file mode 100644
index 0000000..877c2bd
--- /dev/null
+++ b/ONOS_VERSION
@@ -0,0 +1 @@
+2.2-latest