added ssl_verify_mode parameter to chef-client provisioner

Opscode has just released Chef 12.
The default for `ssl_verify_peer` seems to have changed to `ssl_verify_peer` in it from `ssl_verify_none`.
This produces errors like the following when you are using self-hosted Chef-server with default (self-signed) certificate:
```
ERROR: SSL Validation failure connecting to host: chef.yourdomain.com - SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed
```

Setting `ssl_verify_peer` to `ssl_verify_none` fixes the issue.
You can now specify the parameter in Packer config.