Deprecation warnings

Getting these deprecation warnings

- Ubuntu 16.04
- Java agent

```
Deprecated features used!
The on_platforms option to node_map has been deprecated at 1 location:
- /var/chef/cache/cookbooks/powershell/libraries/powershell_module_resource.rb:25:in `'
See https://docs.chef.io/deprecations_internal_api.html for further details.
method access to node attributes (node.foo.bar) is deprecated and will be removed in Chef 13, please use bracket syntax (node["foo"]["bar"]) at 3 locations:
- /var/chef/cache/cookbooks/python/attributes/default.rb:23:in `from_file'
- /var/chef/cache/cookbooks/python/attributes/default.rb:24:in `from_file'
- /var/chef/cache/cookbooks/python/attributes/default.rb:39:in `from_file'
See https://docs.chef.io/deprecations_attributes.html for further details.
```

Start by reading powershell/libraries/powershell_module_resource.rb at line 25 and python/attributes/default.rb at lines 23, 24, and 39, then compare them with the linked Chef deprecation guidance. Confirm the warnings under the reported Ubuntu 16.04 and Java agent setup; done means those deprecation warnings no longer appear.

Written by the indexing model from the issue text.