Replacing ini_get instances with inigetwrapper usages

resolves https://github.com/owncloud/core/issues/11074
getBytes() does not have to be compatible with computerFileSize(). It only needs to able to handle short hand notation actually allowed by PHP. These are defined in http://php.net/manual/en/faq.using.php#faq.using.shorthandbytes. In particular only K, M and G are defined. Also see the examples section in http://php.net/manual/en/function.ini-get.php