Add --update option to deploy create

To allow updating an existing deployment

- Check the deployment dir exists when updating
- Write to temp dir, then safely copy tree
- Don't overwrite data dir or config.env

stack_orchestrator/data/config/test/script.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "Hello"

stack_orchestrator/data/config/test/settings.env

@@ -0,0 +1 @@
+ANSWER=42