Optionally install specific version of Beats

[widhalmt]

A possible solution could be similar to what we did to Logstash in NETWAYS/ansible-role-logstash#75

Two notes, though:

1. If you find a better way: You're very welcome. The ugly string you have to write to use a specific version was chosen to have a solution that will work with rpm and deb versions. Maybe there's a better way.
2. You should add installation of a specific version to Molecule. If you do so, add a test to verify.yml to make sure the right version is actually installed.