Remove EOL operating systems, Legacy facts.

[juniorsysadmin]

• Update the repo_url_suffix table in the README, dropping EOL operating systems and adding new ones
• Drop explicit handling of Debian 6 and 7 (EOL)
• Drop explicit handling of Ubuntu 10.04 and 12.04 (EOL)
• Drop explicit handling of EOL Fedora versions
• Drop explicit handling of RHEL5 (EOL)
• Add explicit handling of Debian 9
• Replace most Legacy facts with Modern facts using the new facts hash syntax. This explicitly drops support for Facter versions less than 3, which are not supported under Puppet4/5 anyway.
• Drop explicit support for Amazon Linux versions less than 2015.03. Amazon Linux is rolling distribution, and earlier versions did not have Facter 3
• Use the Modern system32 fact

Also:

• Remove the old Puppetlabs footer from the README Reference CONTRIBUTING.md instead
• Perform minor doc fixes
• Use less granular error handling

[juniorsysadmin]

I haven't replaced $::lsbdistcodename $facts['os'['distro']['codename'] (to replace $::lsbdistcodename in nodesource/apt.pp) is not handled by rspec-puppet-facts, so will need mocked values.