opensearch: internal_users.yml changes and securityadmin.sh runs on every run #511

hash.sh generates a new bcrypt salt on every run, so the rendered internal_users.yml always differs, the task reports a change and securityadmin.sh runs every time.

Expected: hash only when the password changed, for example by verifying the stored hash against the configured password first.

Found while testing OpenSearch 2.19.7 and 3.9.0 for #508.