Fixes #45: use chef_config_dir on chef bootstrap

Take chef_config_dir into account during bootsrap

Fixes the chef bootstrap spec
Adds http/s proxy configuration option

This change targets provisioning to environments that have proxy-only internet access.

It's possible to an http_proxy option solo.rb but it's required on two
more places:
1. Omnibus/bootstrap script.
2. The command that runs chef-solo itself.

Tested on RHEL only.

Insert the proxy settings _after_ the shebang

Escape the sed replacement string properly
Merge remote-tracking branches 'almog/issue45-bootstrap-chef-config' and 'almog/proxy-rebased-against-locomote-master-squashed'