Use --allow-downgrades to install specific Vagrant version

[jbemmel]

There is a later release, if we want people to use this specific one the user may need to downgrade

[ipspace]

Do we want people to use that specific release?

[jbemmel]

Do we want people to use that specific release?

I don't know, I did not pin the version but I figured it is there for a reason? Vagrant just came out with a new release and the install script stopped working

[ipspace]

OK, let's put this in perspective:

• We're pinning the Vagrant version to 2.4.0-1
• 2.4.1 came out in January (11 months ago)
• 2.4.2 came out on November 2nd quickly followed by 2.4.3 (November 12th).
• Vagrant is usually not included in a Linux distro, so it has to be installed by some installation script.
• Nobody so far found this to be a problem (while we got several reports of Cumulus/Ansible SNAFU)

The big question thus remains: what exactly were you doing to hit this bug and is it relevant to a regular user?

[jbemmel]

OK, let's put this in perspective:

• We're pinning the Vagrant version to 2.4.0-1
• 2.4.1 came out in January (11 months ago)
• 2.4.2 came out on November 2nd quickly followed by 2.4.3 (November 12th).
• Vagrant is usually not included in a Linux distro, so it has to be installed by some installation script.
• Nobody so far found this to be a problem (while we got several reports of Cumulus/Ansible SNAFU)

The big question thus remains: what exactly were you doing to hit this bug and is it relevant to a regular user?

I ran apt-get upgrade -y, it installed the new Vagrant version and removed the vagrant-libvirt plugin which broke my setup. So I ran netlab install vagrant again to correct that - and found that it no longer works if the user has a newer version of Vagrant installed

Given "only submit fixes for what you tested" and "we don't want to publish/generate something that doesn't work" I decided to submit this PR, which I believe would be relevant to anyone who has a later version of Vagrant installed

BTW we're technically not "pinning", we're installing a specific version. We could truly pin it and prevent the upgrade

[ipspace]

It might be more productive to start with "An upgrade broke my Vagrant setup and netlab install libvirt could fix it... but we need the --allow-downgrades option to make it work"... or not 🤷‍♂️