Added initializer to load all drivers when Deltacloud is booting

This should fix all the nasty threading issues we recently uncovered, since there is not need for calling 'require' under threading non-safe context (since all drivers are loaded when DC is booting).

NOTE: This will increase DC boot time, but it is not horrible ;-)

NOTE: Only drivers with YAML config files (config/drivers/*.yaml) are loaded. This affect the RPM version of Deltacloud, where the drivers that are not installed will not be loaded.