[A recent breaking change upstream in Golang's crypto
library](https://github.com/golang/crypto/commit/e4e2799dd7aab89f583e1d898300d96367750991)
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.
In AWS we can derive the `VpcId` and AZ from the `SubnetId`, so now we do. In the config you can now only specify the `SubnetId`.
This fixes issue #4693.
This commit substitutes the now-deprecated gosdc library for the newer
triton-go library. This is transparent from a user perspective, except
for the fact that key material can now be ommitted and requests can be
signed with an SSH agent. This allows for both encrypted keys and ECDSA
keys to be used.
In addition, a fix is made to not pass in an empty array of networks if
none are specified in configuration, thus honouring the API default of
putting instances with no explicit networks specified on the Joyent
public and internal shared networks.
JD Friedrikson
Matthew Hooker
numa
Rémi Jouannet
Luke Farnell
Paul Thrasher
Ben Fagin
luis-silva
Sean Chittenden
Justin Reagor
James Nugent
Christopher Boumenot
Rickard von Essen