ssh key added rather than change on VM

Using this to change the ssh key of the VM.
```
linuxVM.update()
.defineNewExtension("myextension")
.withPublisher("Microsoft.OSTCExtensions")
.withType("VMAccessForLinux")
.withVersion("1.4")
.withProtectedSetting("username","centos")
.withProtectedSetting("ssh_key",sshkey)
.withProtectedSetting("reset_ssh", "true")
.attach()
.apply();
```

This is appending the as new public key rather than update the existing key.

keys are appended ~/.ssh/authorized_keys.
I'm able to do ssh from both the key.