Add local.sh support and samples of local.sh and locarc

Run $TOP_DIR/local.sh at the end of stack.sh if it exists and is executable.
This allows the user to automatically perform local actions on every re-stack,
such as creating custom flavors or specific tenants/users.  Like localrc, this
file is not distributed with DevStack so user modifications will be undisturbed.

Add local.sh to .gitignore

Examples of local.sh and localrc are in the samples/ directory.

Change-Id: I0be6b4d80ce084981cac8a3a8f1dc9bc8c3bbd4e
diff --git a/stack.sh b/stack.sh
index 3a7fc5d..fd2daaf 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1706,6 +1706,16 @@
 fi
 
 
+# Run local script
+# ================
+
+# Run ``local.sh`` if it exists to perform user-managed tasks
+if [[ -x $TOP_DIR/local.sh ]]; then
+    echo "Running user script $TOP_DIR/local.sh"
+    $TOP_DIR/local.sh
+fi
+
+
 # Fin
 # ===