Adds ability to have per-class configuration and special properties
with usage "Config". Such properties get their values from config
(if it is present) rather than from object model.
Config files can also modify defaults for other property types.
Config files are stored in special folder that is configured in
[engine] section of Murano config file under class_configs key.
Config files must me named using %FQ class name%.json or
%FQ class name%.yaml pattern and contain dictionary of a form
propertyName -> propertyValue
Change-Id: I0f45fa7064183f5605c5ef393b5b00e8c8ae2bda
Implements: blueprint class-configs
b5f0b0f245