[gantree] allow config through .gantree.cfg

<div><p>Allow defaults for commands to be set in a json file<br></p>
<pre>
<code>{
  "ebextensions" : "configs/.ebextensions",
  "default_instance_size" : "m3.medium"
}</code>
</pre></div>

https://bleacherreport.lighthouseapp.com/projects/6296/tickets/17575-gantree-allow-config-through-gantreecfg
can we use something like:
Rails.root.join('config', 'gantree.json')