Improve install_cert.sh: validation, backup, and UX enhancements

## Suggestions for install_cert.sh improvements

1. **File existence and format validation**
- Check that ${DOMAIN}.pem, ${DOMAIN}.key, and ${DOMAIN}.ca-cert exist and are non-empty before proceeding.
- Optionally, use openssl to verify that the certificate and key match (modulus check).

2. **Automatic route backup**
- Before making any changes, export the current route YAML to a timestamped backup file (e.g., using: oc get route -o yaml > backup).
- This allows quick rollback if something goes wrong.

3. **Safer route replacement**
- If the route already exists, handle it gracefully (e.g., use oc replace, oc apply, or delete the old route first).
- Echo a clear error message if route creation fails.

4. **CA chain handling**
- If the CA chain is split across multiple files, concatenate them into ${DOMAIN}.ca-cert before running the script.

5. **User experience improvements**
- Echo the full oc command before running it so the user can see exactly what will be executed.
- Add clear error messages and confirmations at each step.

These changes will make the script safer, more robust, and easier to use and recover from errors.

Start by reading install_cert.sh and trace how it handles the ${DOMAIN}.pem, ${DOMAIN}.key, ${DOMAIN}.ca-cert files and the oc route command. Check the available openssl validation and route YAML export steps, then verify that existing routes, failures, backups, confirmations, and CA-chain inputs are handled clearly and safely.

Written by the indexing model from the issue text.