Read sudo_prefix from config file

This is first step to allow to overwrite some `Fabric` default values via config file. I personally needed to set [sudo_prefix](http://docs.fabfile.org/en/1.5/usage/env.html#sudo-prefix) in config file:

```
[ssh]
sudo_prefix =sudo -E -S -p '%(sudo_prompt)s'
```

The additional `-E` switch for `sudo` preserves environment variable which in my case was `SSH_AUTH_SOCK` that I needed so that `chef` user can log into another machines using `ForwardAgent` ssh option when running one of the recipe.