Refs: #2507 (comment)

Attempting to run ansible against our three centos7-x64 hosts results in errors, detailed below.

• release-digitalocean-centos7-x64-1
• test-rackspace-centos7-x64-1
• test-softlayer-centos7-x64-1

It also appears that the three hosts were set up at different points in time and currently do not have the same packages installed (see below about the git2u and devtoolset-6-libatomic-devel packages).

Failed 'baselayout : centos7 | install ius'

Reported in #2531

The "centos*.iuscommunity.org" URLs were retired in 2020: iusrepo/announce#18
The URL can be replaced by https://repo.ius.io/ius-release-el7.rpm, which fixes this problem but then leads to...

No package matching 'git2u' found available, installed or updated

On test-rackspace-centos7-x64-1 and release-digitalocean-centos7-x64-1 (it's already installed on test-softlayer-centos7-x64-1).
The git2u package was renamed to git216 in December 2019 (iusrepo/git216#14) and then made End-of-Life in May 2020 (iusrepo/announce#19). git222 looks like the current replacement.
git222 conflicts with git2u on test-softlayer-centos7-x64-1 and with git216 on test-rackspace-centos7-x64-1. Sam seemed to have noticed this in fa08ea2.

No package matching 'devtoolset-6-libatomic-devel' found available, installed or updated

On test-softlayer-centos7-x64-1 and release-digitalocean-centos7-x64-1 (it's already installed on test-rackspace-centos7-x64-1.
Perhaps it's no longer required, at least in the default configuration CI builds? (Apparently it's needed when building with clang nodejs/node#30093).
It appears that devtoolset-6 is no longer available in Software Collections: https://forums.centos.org/viewtopic.php?t=71663
This means we cannot easily install/reinstall it via ansible/yum. Perhaps due to the way devtoolsets work this isn't a problem in practice and we can use a later one (e.g. devtoolset-8)? In the past we've been very conservative about switching.