Make install-openappstack.sh idempotent

After hitting an install error (#928 (closed)) I re-ran install-openappstack.sh but it breaks with namespaces "oas" already exists :

❯ ./install/install-openappstack.sh                     
✚ generating manifests
✔ manifests build completed
► installing components in flux-system namespace
◎ verifying installation
✔ source-controller: deployment ready
✔ kustomize-controller: deployment ready
✔ helm-controller: deployment ready
✔ install finished
Tracking branch 924-document-seperation-of-k8s-cluster-and-flux-installation for https://open.greenhost.net/openappstack/openappstack flux repo
Error from server (AlreadyExists): namespaces "oas" already exists

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Make install-openappstack.sh idempotent