It appears that ruby-2.7 (which ships with Chef 16) likely doesn't like
``0022`` as an integer and is causing issues on Chef 16. To workaround the
issue, we can simply change this to a string.

The following is the output of the original error I've been hitting.

```
Recipe: rabbitmq::management_ui
  * rabbitmq_plugin[rabbitmq_management] action enable
    * execute[rabbitmq-plugins enable rabbitmq_management] action run

      ================================================================================
      Error executing action `run` on resource 'execute[rabbitmq-plugins enable rabbitmq_management]'
      ================================================================================

      TypeError
      ---------
      no implicit conversion from nil to integer

      Resource Declaration:
      ---------------------
      # In /tmp/kitchen/cache/cookbooks/rabbitmq/providers/plugin.rb

37:     execute "rabbitmq-plugins enable #{new_resource.plugin}" do
38:       umask 0022
39:       Chef::Log.info "Enabling RabbitMQ plugin '#{new_resource.plugin}'."
40:       environment shell_environment.merge(
41:         'PATH' => "#{ENV['PATH']}:/usr/lib/rabbitmq/bin"
42:       )
43:       new_resource.updated_by_last_action(true)
44:     end
45:   end
```

Signed-off-by: Lance Albertson <lance@osuosl.org>