Print kernel version

Adding `uname -a` to stack.sh to make easy to see from the devstack logs
what was the actually running kernel version.

Change-Id: I0068504bf055a588b155b0a60215440d365bf53e
diff --git a/stack.sh b/stack.sh
index 9069367..79f8fa3 100755
--- a/stack.sh
+++ b/stack.sh
@@ -89,6 +89,9 @@
     exit 1
 fi
 
+# Print the kernel version
+uname -a
+
 # Prepare the environment
 # -----------------------