This change leads to expected exceptions when bootstrapping the
application since the \Icinga\Application\Config class was
refactored but its usage not.
refs #4354
refs #4353
Added the clearstatcache call before enabling or disabling modules,
as this prevents a 'File exists' error that occurs sometimes on
symlink creation (even when the folder is empty). Also added tests
for teh moduleManager
refs #4092