Allow default model setting per database specification

This change is to allow per db spec default model setting.

Although the option such as 'use_transactions' can be specified by model, the option goes with each connection definition. Any model that refers to that connection should get the setting without any code.

sample:

~~~
default: &default
  adapter: informix
  <blah blah>
  default_model_settings:
    use_transactions: false
~~~