fix(contrib/ec2): add json extension to cloudformation template

The CloudFormation Template has a .template extension although its a json file.
This makes it difficult to parse/edit it as editors dont recognize it as JSON file.
So added the .json extension behind its name and also updated gen-json.py which
uses it.

contrib/ec2/gen-json.py

@@ -2,7 +2,7 @@
 import json
 import os
 
-template = json.load(open("deis.template",'r'))
+template = json.load(open("deis.template.json",'r'))
 
 with open('../coreos/user-data','r') as f:
   lines = f.readlines()