YAML file load split in new dedicated class

The process of loading YAML file has been moved out of Configuration class into
new dedicated ConfigurationLoader class. As of now Configuration class does not
refer to YAML file format anymore.
check memory is either int or string

If the type of parsed memory value is different, raise exception.
add new __parse_cpu() in ConfigurationDomain class
add domain name in exception messages

This may help users finding which part of the configuration file the
error comes from.
use dependency injection in Configuration

Configuration class does not depend anymore on ConfigurationLoader.
Dependency injection is used by passing a ConfigurationLoader instance
as parameter of Configuration __init__().