Treat the forwarder user as a separate parameter

[Tamerz]

Pull Request (PR) description

Now the forwarder uses a completely different user name. I thought using a new splunk::params::splunk_forwarder_user would make things cleaner.

It also sets the forwarder user to the correct default user for Windows.

This Pull Request (PR) fixes the following issues

Fixes #369

[kenyon]

Converting params.pp to hiera data would simplify this, but that's a separate task. LGTM once REFERENCE.md is updated so that the tests can run. bundle exec rake strings:generate:reference

Also, please rebase on master to remove the merge commit. Thanks.