Fixed PHP warnings in Installer.

install/dist/conf/centos52.conf.php

install/dist/conf/centos53.conf.php

install/dist/conf/centos70.conf.php

install/dist/conf/centos72.conf.php

install/dist/conf/centos80.conf.php

install/dist/conf/debian100.conf.php

install/dist/conf/debian110.conf.php

install/dist/conf/debian40.conf.php

install/dist/conf/debian60.conf.php

install/dist/conf/debian90.conf.php

install/dist/conf/debiantesting.conf.php

install/dist/conf/fedora32.conf.php

install/dist/conf/fedora33.conf.php

install/dist/conf/fedora9.conf.php

install/dist/conf/gentoo.conf.php

install/dist/conf/opensuse110.conf.php

install/dist/conf/opensuse112.conf.php

install/dist/conf/ubuntu1604.conf.php

install/dist/conf/ubuntu1710.conf.php

install/dist/conf/ubuntu1804.conf.php