Switch orbi to my mirror of the PHP PPA

Many of our servers use this Ubuntu PPA to get current PHP packages: https://launchpad.net/~ondrej/+archive/ubuntu/php

However, orbi is still on Ubuntu Xenial, and that PPA doesn't have Xenial packages anymore. This means the PHP packages are not installable from the repository anymore. "apt reinstall php5.6-fpm" will fail, we can't rebuild orbi if it was needed, and we can't run the KDE Identity vagrant VM.

I mirrored the old xenial packages on my own server, and changed the php role to use it on orbi. I tested that this works on the vagrant VM. It shouldn't cause any functional change on the actual orbi server.

It's not ideal that this APT repository is on my server; any suggestions for where in KDE infra we could put it? Although this is temporary anyway, hopefully we'll upgrade orbi away from xenial soon enough...