make ConfigurationManager#load_dir lazy

[doudou]

ConfigurationManager#load_dir currently loads all configuration files present in the given folder. This causes loading of the underlying orogen models (for config file validation) and:

• causes some scripts to be unusable because a config file is not up-to-date, even if said script does not use this config file
• is a major performance hit (a few seconds, can be up to 10s on "cold cache")