fix ImmutableAttributeModification Exception

Running this recipe under Chef-11.8 I get this:

```
Chef::Exceptions::ImmutableAttributeModification
------------------------------------------------
Node attributes are read-only when you do not specify which precedence level to set. To set an attribute use code like `node.default["key"] = "value"'


Cookbook Trace:
---------------
  /var/chef/cache/cookbooks/jmxtrans/recipes/default.rb:23:in `block in from_file'
  /var/chef/cache/cookbooks/jmxtrans/recipes/default.rb:22:in `each'
  /var/chef/cache/cookbooks/jmxtrans/recipes/default.rb:22:in `from_file'

Relevant File Content:
----------------------
/var/chef/cache/cookbooks/jmxtrans/recipes/default.rb:

 16:  end
 17:  
 18:  user node['jmxtrans']['user']
 19:  
 20:  # merge stock jvm queries w/ container specific ones into single array
 21:  servers = node['jmxtrans']['servers']
 22:  servers.each do |server|
 23>>   server['queries'] = node['jmxtrans']['default_queries']['jvm']
 24:    case server['type']
 25:    when 'tomcat'
 26:      server['queries'] << node['jmxtrans']['default_queries']['tomcat']
 27:    end
 28:    server['queries'].flatten!
 29:  end
 30:  
 31:  ark "jmxtrans" do
 32:    url node['jmxtrans']['url']
```