This migrates the CentOS guest plugin to identify machines by
what is in os-release rather than based on if the /etc/centos-release
file exists.
It just so happens that the Rocky Linux plugin is set up to inherit
directly from :redhat (not :centos) and thus the CentOS flavor is
attempted before Rocky in the depth first search. For some reason
the rockylinux/8 vagrant box still has the /etc/centos-release file
in it even though it's only supposed to inherit from RHEL.
The almalinux/9 box does not have /etc/centos-release.
