Unify the two vagrant scripts for Ubuntu 18. The script can now
be run in three modes: no webserver, with apache, with nginx.
The default mode is to not install any webserver at all. This is
normally sufficient when just developping.
The commit also switches from bento to generic boxes and adds config
for running with a libvirt provider. You need an NFS deamon for
synchronized folders.
The instructions in
[`VAGRANT.md`](42c80893cb/VAGRANT.md)
still work as before. The names of the Vagrant machines are updated so
that Ubuntu 18.04 (previously called `ubuntu`) is now called `ubuntu18`
and Ubuntu 20.04 is now called `ubuntu20`.
The version changes from Ubuntu 18.04 to Ubuntu 20.04 are:
- Python: 3.6 to 3.8
- Postgres: 10 to 12
- PHP: 7.2 to 7.4
In the `apt-get`, I changed `--force` to `--allow-downgrades --allow-remove-essential --allow-change-held-packages`, because the former is deprecated. Cf. the [manpage of apt-get](http://manpages.ubuntu.com/manpages/focal/man8/apt-get.8.html)
The php module `codesniffer` was previously installed via Composer, but it is available via the Ubuntu repository, so I installed it via `apt-get` now.
Basically leaves the part that needs to be done by the automatic
vagrant script in the bash script and moves the remaining part
into an OS-independent md file.
Also fixes some of the instructions for CentOS. Now almost runs
through (some minor issue with permission remains).
