[A recent breaking change upstream in Golang's crypto
library](e4e2799dd7)
has broken SSH connectivity for a few builders:
```
==> qemu: Waiting for SSH to become available...
2017/05/20 16:23:58 ui: ==> qemu: Waiting for SSH to become available...
2017/05/20 16:23:58 packer: 2017/05/20 16:23:58 [INFO] Attempting SSH connection...
2017/05/20 16:23:58 packer: 2017/05/20 16:23:58 reconnecting to TCP connection for SSH
2017/05/20 16:23:58 packer: 2017/05/20 16:23:58 handshaking with SSH
2017/05/20 16:23:58 packer: 2017/05/20 16:23:58 handshake error: ssh: must specify HostKeyCallback
2017/05/20 16:23:58 packer: 2017/05/20 16:23:58 [DEBUG] SSH handshake err: ssh: must specify HostKeyCallback
2017/05/20 16:24:05 packer: 2017/05/20 16:24:05 [INFO] Attempting SSH connection...
2017/05/20 16:24:05 packer: 2017/05/20 16:24:05 reconnecting to TCP connection for SSH
2017/05/20 16:24:05 packer: 2017/05/20 16:24:05 handshaking with SSH
2017/05/20 16:24:05 packer: 2017/05/20 16:24:05 handshake error: ssh: must specify HostKeyCallback
2017/05/20 16:24:05 packer: 2017/05/20 16:24:05 [DEBUG] SSH handshake err: ssh: must specify HostKeyCallback
```
Specifying HostKeyCallback as insecure should make things work again
and would make sense for packer's use case.
The null builder is not really a bulider, it just setups a SSH
connection and runs the provisioners. It can be used to debug
provisioners without incurring high wait times. It does not create any
kind of image or artifact.