Fixes for the Stash library

##### 1. Default settings should not be specified in attribute files 
`Chef::Recipe::Stash.settings` method is going to set default settings:
```
settings ||= node['stash']
settings['database']['port'] ||= default_database_port settings['database']['type']
settings['database']['testInterval'] ||= 2
```
But it will never be done because the node will always have these attributes specified in `attributes/default.rb`. 
For example, even if we set `['database']['type'] = 'postgresql'`, the port will be "3306" anyway. So that, these attributes should not be specified in attribute file.

##### 2. `default_database_port ` method is unavailable
This method should be a class method, instead of an instance method. Otherwise, it can not be called from `Chef::Recipe::Stash.settings` method 

##### 3. `settings` should be a hash.
While copying `settings ||= node['stash']` it should be also converted to Hash object, becouse Node object is read-only and overriding below in unavailable.