acceptance test docs do not match reality

The instructions in https://github.com/puppetlabs/puppet/blob/master/acceptance/README.md do not match up with reality.

• The "Preserving Hosts" section references a rake task (ci:test_against_preserved_hosts) that does not exist
• the configs in https://github.com/puppetlabs/puppet/tree/master/acceptance/config/nodes are half EOL systems, and are missing all the current systems (and should be replaced with beaker-hostgenerator, I assume, making all references to CONFIG= in the README obsolete
• The documentation for testing against the local changes (arguably the most common usecase for a developer) is buried under statements like "See notes on running acceptance with Vagrant for more details on using a local git daemon."

This ticket is sponsored by the struggles in https://github.com/puppetlabs/puppet/pull/6831