Improvements to vagrant/chef to make more customizable

This PR is related to issue #9 and composed of:
* Fix proxy's bug
* Allow using a solo.local.json to customize solo.json
* Define vagrant_common to use on cluster and standalone Vagrantfiles.

## About proxy bug
On old Vagrantfile, the proxy session doesn't work because  Vagrant don't support two calls on provision block. More info here: http://stackoverflow.com/questions/13065576/override-vagrant-configuration-settings-locally-per-dev

## About solo.local.json
Now it's possible to overwrite solo.json with solo.local.json. Basically, we use this solo.local to change openruko's home and to reject some chef recipes we don't need using `reject_run` field.

## About vagrant_common.rb
It's the way we found to share common helpers on both cluster and standalone Vagrantfiles.

## Note

Guys, i found this that can help us, https://github.com/puppetlabs/puppetlabs-openstack_dev_env. This is Openstack's builder, using puppet. We can extract some ideas from their Vagrantfile to help us improve local customizations.