Allow chef omnibus install.sh url to be configurable

Inside ssh_base.rb, the url for the chef omnibus install.sh is currently hardcoded to https://www.opscode.com/chef/install.sh. This should be configurable to support custom variations that might be needed (such as in a corporate network, etc). This will also allow the possibility of removing an external dependency on opscode.com.

This pull request allows the url to be configured in the .kitchen.yml (example below)

driver config:
  require_chef_omnibus: true
  chef_omnibus_url: "https://example.com/chef/install.sh"