Instead of having to update these flavor.rb files every time a
new version of CentOS/RHEL come out let's dynamically pick up
the version (7,8,9) from the machine.
This was inspired slightly by the Rocky Linux guest flavor.rb.
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.
