renames internal attribute name from `method` to `init_method`

[vitalis]

• Not a breaking change

• Right now the cookbook fails to compile with Chef13:

     ================================================================================
     Recipe Compile Error in /tmp/kitchen/cache/cookbooks/logstash/resources/service.rb
     ================================================================================
  
     ArgumentError
     -------------
     Property `method` of resource `logstash_service` overwrites an existing method.
  
     Cookbook Trace:
     ---------------
       /tmp/kitchen/cache/cookbooks/logstash/resources/service.rb:14:in `class_from_file'