Symbiosis: PHP 7.1/7.2 support
Imported from https://www.github.com/BytemarkHosting/symbiosis/issues/128
(https://forum.bytemark.co.uk/t/symbiosis-stretch-update/2848/30)
PHP7.0's EOL is 3/12/18, which is a little short, given that PHP7.2 is only 'officially' available in Buster, and PHP7.1 in Sid.
We already run 7.1 and 7.2 in containers (Systemd-nspawn, Docker) for some customers using Symbiosis, but the setup procedure is mostly manual.
Ideally we'd want to be able to create a file e.g. /srv/domain.com/config/php containing 7.1< or 7.2 to automatically use the appropriate Apache template for that domain.