hosting: ensure that php5-fpm is installed before any php app

By collecting various dependencies around applications into a single
block and putting in restrictive dependencies, this ensures that no
unintended php implementation pulls in apache2.
hosting: implement admin user flag

This enables all admin users to run commands as the main admin user.

Additionally it installs a systemctl_hosting script which automatically
switches to the main user's session to manipulate the user instance of
systemd.