Fixed deprecated variables in *.erb template

The usage of variables in erb template for ssmtp config is deprecated. 

```
puppet apply --noop --show_diff --modulepath=/tmp/puppet/puppet-modules/ /tmp/puppet/manifests/site.pp
Warning: Variable access via 'mailhub' is deprecated. Use '@mailhub' instead. template[/tmp/puppet/puppet-modules/ssmtp/templates/ssmtp.conf.erb]:13
[...]
```

Just added some `@` ;) 

Thank you