1804: issues regarding puppet repo deb removing versions

[aerickson]

In #219 (first production commit for 18.04), the puppet code installed the unversioned puppet release deb (https://apt.puppetlabs.com/puppet-release-bionic.deb).

Around #285, the https://apt.puppetlabs.com/puppet-release-bionic.deb eliminated the 6.x versions and switched to 7.0. This caused an unexpected upgrade... so we pinned puppet to 7.5 in #285.

Imaging was broken as of #285, but discovered in https://bugzilla.mozilla.org/show_bug.cgi?id=1711763. The code would try to install puppet 7 before the repo was installed and apt updated.

#312 tried to fix this but it caused a new problem. It installed the puppet7-release deb, but that just caused later convergence issues when the puppet code installs the puppet-release deb (they can't coexist) and code wasn't added to remove the puppet7 releasee deb.

I found the two nodes I had been testing with stuck in Puppet loops.

Error: /Stage[main]/Linux_packages::Puppet/Package[puppet repo deb]/ensure: change from 'purged' to 'present' failed: Execution of '/usr/bin/dpkg --force-confold -i /tmp/puppet.deb' returned 1: dpkg: regarding /tmp/puppet.deb containing puppet-release:
 puppet-release conflicts with puppet7-release
  puppet7-release (version 7.0.0-2bionic) is present and installed.

dpkg: error processing archive /tmp/puppet.deb (--install):
 conflicting packages - not installing puppet-release

The bootstrap script has always installed a versioned deb and the puppet code has removed the versioned deb and installed the unversioned (a problem due to the versions disappearing).

Problem: When puppet 8 comes out, the current code will likely break. Maybe we'll be off 18.04 by the point puppet 8 comes out.

Proposal: Always install the versioned deb (the only immutable option) in the puppet code and bootstrap.

Complications: The versioned and non-versioned release debs are incompatible, so sequencing could be tricky.