Adjust class admin_setting_configmultiselect_modules to include a default

In MDL-27123 a new admin class admin_setting_configmultiselect_modules was created that used lazy loading to help improve performance.

Unfortunately I did not include the use of a default value for this class as it was not needed at the time. Of course I now do have a need to use this class with a default value. Yes it is my own fault I know!