Dynamic UI optimization (refactoring)

Currently, all form definition parsing, fields creation and form instantion are done in ServiceConfigurationForm instance __init__ method, which was not good indeed. This was done partly due to a need to rapidly develop and debug dynamic_ui, partly due to the opinion that using properties and attribute mappings (both of them are stored inside the instantiated form) prevents separating form layout creation from instantiating this form.

It is very desirable to separate form's fields creation (i.e. form class definition) from form instantiation, because form is instantiated much more often (by django) than its class is defined.