Docker homebridge ssh "shutdownCommand" doesn't appear to function correctly.
### Backstory
I've recently migrated my homebridge installation to now using docker which is running on a raspberry pi. I was previously running homebridge in a Freenas jail and using homebridge-wol but this was running as root; now docker homebridge is running as user "abc". On Freenas I could successfully both wake on lan and shutdown my win10 pc.

The homebridge-config-ui (v4.7.0) now has an option to enter the terminal of the running homebridge docker instance and from there I had the ability to generate new ssh keys for the user "abc". From this terminal, I can ssh into my win10 pc and also issue a command to shut down my windows pc over ssh. I generated my keys as per:

https://github.com/oznu/docker-homebridge/issues/113

i.e. ssh -i /homebridge/.ssh/id_rsa simon@simon-pc shutdown /s /f /t 0

the above command successfully shuts down my win10 pc from the terminal. 

### Issue
However, issuing the very same command using the homebridge-wol config now fails:

i.e. "shutdownCommand": "ssh -i /homebridge/.ssh/id_rsa simon@simon-pc shutdown /s /f /t 0"

### Environment
* os & os version: Raspberry PI docker  image.
* node version: v12.14.1
* homebridge version: Version 0.4.50
* homebridge-wol version: v4.1.2
* homebridge-config-ui (v4.7.0) 
### Configuration
My configuration looks like this:
```
  {
            "accessory": "NetworkDevice",
            "name": "simon-pc",
            "mac": "XX-XX-XX-XX-XX-XX",
            "broadcastAddress": "172.16.28.254",
            "wakeGraceTime": 0,
            "shutdownGraceTime": 0,
            "ip": "172.16.28.6",
            "shutdownCommand": "ssh -i /homebridge/.ssh/id_rsa simon@simon-pc shutdown /s /f /t 0"
        }

### Log

[1/12/2020, 2:57:20 PM] [simon-pc] An error occured while trying to shut down "simon-pc" (172.16.28.6): Error: Command failed: ssh -i /homebridge/.ssh/id_rsa simon@simon-pc shutdown /s /f /t 0
Host key verification failed.

    at ChildProcess.exithandler (child_process.js:295:12)
    at ChildProcess.emit (events.js:223:5)
    at maybeClose (internal/child_process.js:1021:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:283:5) {
  killed: false,
  code: 255,
  signal: null,
  cmd: 'ssh -i /homebridge/.ssh/id_rsa simon@simon-pc shutdown /s /f /t 0',
  stdout: '',
  stderr: 'Host key verification failed.\r\n'
}

The wake up (wol) components continue to function correctly.
