Thanks @Papaul great work!

So it seems I was wrong we can run homer on these without having to add anything to devices.yaml for now. Just need to set to 'active' in Netbox and run Homer.

I did that and it worked fine on ssw1-a1-codfw. Unfortunately for the others it didn't work. The Leaf switches all failed as follows:

Type "yes" to commit, "no" to abort.
> yes
INFO:homer.transports.junos:Committing the configuration on lsw1-b8-codfw.mgmt.codfw.wmnet
ERROR:homer:Failed to commit on lsw1-b8-codfw.mgmt.codfw.wmnet: Commit error: 
configuration check-out failed

ERROR:homer:Homer run had issues on 14 devices: ['lsw1-a1-codfw.mgmt.codfw.wmnet', 'lsw1-a2-codfw.mgmt.codf.....................

ssw1-a8-codfw caused a bit of an issue at first, I realised as it's the one me and volans were testing before and as a result had the old RSA key for the homer user. Fixed that via console at it was fine.

I've set all back to 'planned' in Netbox for now. Will need to investigate what the Leaf switches don't like.