Avoid double-newlines in salt-call output

When using the salt provisioner with verbose=true, most lines read with an extra newline:
```
[INFO    ] Syncing modules for environment 'base'

[INFO    ] Loading cache from salt://_modules, for base)

```

because the line read has a newline, and emitting the log entry again includes an additional newline.
Merge pull request #4900 from jquast/patch-1

Avoid double-newlines in salt-call output