adding comments, remove default attr setter
adding comments, remove default attr setter

I removed default attr setter for `default['mongodb3']['config']['mongod']['storage']['engine']` in order to keep same behaiviour with mongoldb-org-server package
```
if pkg_major_version >= 3.2
  default['mongodb3']['config']['mongod']['storage']['engine'] = 'wiredTiger' # default since 3.2 : wiredTiger
else
  default['mongodb3']['config']['mongod']['storage']['engine'] = 'mmapv1' # default until 3.2 : mmapv1
end
```

sunggun-yu/chef-mongodb3,sunggun-yu/chef-mongodb3,sunggun-yu/chef-mongodb3,shortdudey123/chef-mongodb3,shortdudey123/chef-mongodb3,shortdudey123/chef-mongodb3