Backport sessionclean from Debian package

Backport the PHP5 sessionclean script and cron job from the latest
Debian package. This replaces the potentially buggy sessionclean
implementation packaged with PHP 5.5 in Ubuntu 14.04. Hiera can also be
used to completely remove the sessionclean script and cron job by
setting 'php::sessionclean::ensure: absent' in a hiera file.

Bug: 71645
Change-Id: I82b5628b4bc7652c57bdf77415c277df7828866f