New build logic for [AETHER-778], [AETHER-779] and [AETHER-782]

- Introduces 2 different tags for tost and tost-onos image
- Stable tag tracks stable commits; master tag tracks ONOS master and tip of the apps
- Updates README and help
- Deprecates the use of the VERSION file
- Uses actual hash for the docker labels

By default DOCKER_TAG points to stable

Change-Id: I7bdb2177b577065a1f4345248c406dcb41ba52a2
diff --git a/Dockerfile.tost b/Dockerfile.tost
index 5345b6e..984d1bb 100644
--- a/Dockerfile.tost
+++ b/Dockerfile.tost
@@ -14,9 +14,11 @@
 # limitations under the License.
 #
 
+ARG DOCKER_TAG
+
 # We start from an onos image and install the apps.
 # We need at least an onos image built with 'tost' profile
-FROM tost-onos as install
+FROM tost-onos:${DOCKER_TAG} as install
 
 ARG KARAF_VERSION
 ARG LOCAL_APPS
@@ -44,8 +46,10 @@
 RUN chmod 755 ./app-install.sh
 RUN ./app-install.sh
 
+ARG DOCKER_TAG
+
 # Create the final image coping over the installed applications from the install stage
-FROM tost-onos
+FROM tost-onos:${DOCKER_TAG}
 
 ARG KARAF_VERSION