Add a util function running_on_slaveserver() to improve code readablility

server/lib/app.inc.php

server/lib/classes/backup.inc.php

server/lib/classes/cron.d/500-backup_mail.inc.php

server/lib/classes/cron.d/550-bind_dnssec.inc.php

server/lib/classes/modules.inc.php

server/plugins-available/apache2_plugin.inc.php

server/plugins-available/backup_plugin.inc.php

server/plugins-available/mail_plugin.inc.php

server/plugins-available/nginx_plugin.inc.php