[AETHER-544][AETHER-523] Improve versioning of `tost` docker image and simplify build workflow

VERSION file is used for the release/tag process of `tost`.

Makefile.vars file defines the stable versions of each component
to be used during the build process.

Dockerfile.tost defines a docker label for each component
and uses the *_VERSION vars to set the docker labels.

Moves build logic inside app-build.sh. Build process is stopped if there are pending
changes in the repositories

Adds check-scripts target for shellcheck verify

Add fetch targets to update the repos

Deprecates the use of git-review for git fetch ref/changes/

Updates README to explain the new build process

Change-Id: I9a3f4b1b67064cd55c755330deaf6efef19ced22
diff --git a/mvn_settings.sh b/mvn_settings.sh
index 6e95aff..b2e72e0 100755
--- a/mvn_settings.sh
+++ b/mvn_settings.sh
@@ -1,4 +1,5 @@
 #!/bin/bash
+
 #
 # Copyright 2020-present Open Networking Foundation
 #
@@ -14,6 +15,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+
+http_proxy=${http_proxy:-}
+https_proxy=${https_proxy:-}
+no_proxy=${no_proxy:-}
+
 if [ -f mvn_settings.custom.xml ] ; then
   cp mvn_settings.custom.xml mvn_settings.xml
   exit 0