Move most of code for dynamic UI form creation into metaclass.

Previously, all form definition parsing, fields creation and form
instantion were 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, 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.

After some thoughts a proper way to separate them was found:
* move code that parses UI definitions, creates fields and widgets for
  a ServiceConfigurationForm subclass into a metaclass (called
  DynamicFormMetaclass);
* split property object creation into 2 phases: create RawProperty
  object in metaclass, while finalizing it (creating actual property
  object) once the form is instantiated;
* completing all assignments and calls that need a form instance in
  ServiceConfigurationForm __init__ method.

Creating form's fields layout during form class definition allows to
speed things a bit up, because form class is redefined only when
metadata is invalidated, while inside Django forms are instantiated
much more often.

Implements: blueprint dynamic-ui-optimization
Change-Id: I9b2617527b410abb7c60df978f9c00f7cef491d3
Minor refactoring of dynamic_ui.

Move more functions to dynamic_ui.helpers.

Change-Id: Ib578a24159dda4de5fecf5df35ff71bc7d704215
Merge "Move most of code for dynamic UI form creation into metaclass."
Merge "Minor refactoring of dynamic_ui."