Set default proxy configuration for Chef provisioners

Set proxy settings for all Chef provisioners based on
`config.proxy.http` etc. The provisioner specific configuration
still takes precedence.

Chef configuration wants the (optional) username and password in
separate config keys, so we need to extract them from our URI.
Document configuring the Chef provisioners
Add unit tests for Config::Proxy class
Add unit tests for Action::ConfigureChefProxy.configure_chef
Document Action::ConfigureAptProxy helper methods
Merge pull request #21 from tmatilai/chef-provisioners

Set default proxy configuration for Chef provisioners

Fixes #19