When running install-sensor-proxy.sh on multiple nodes in a cluster, each
installation was removing all existing pulse-managed-key entries before
adding its own. This caused the following scenario:
1. Run script on node A: node A's key is added to all nodes
2. Run script on node B: node B's key replaces node A's key on all nodes
3. Result: node A's proxy can no longer SSH to other nodes
The fix changes the behavior to:
- Check if the specific SSH key already exists on the target
- Only add the key if not present (idempotent)
- Never remove existing pulse-managed-key entries
This allows multiple sensor-proxy installations to coexist in a cluster,
with each node's proxy key authorized on all nodes.
Related to #738
973f1f9866