Add deprecation message to run_tests.sh

We should finally get rid of run_tests and just move to tox. Adds a
deprecation warning to give this change more visibility.

Change-Id: I4cfd8f031e49ae302ceee18496cc68bcec42c821
Implements: blueprint enhance-tox

run_tests.sh

@@ -1,5 +1,10 @@
 #!/bin/bash
 
+echo ""
+echo "The run_tests script is deprecated and will be removed in the "
+echo "Queens Release Cycle (13.0), in favor of tox."
+echo ""
+
 set -o errexit
 
 function usage {