Attempt to assign a value to unknown setting :trusted_server_facts

[DavidS]

When testing against current puppet master (future puppet6), rspec-puppet fails with this stack trace:

     1.1) Failure/Error: raise ArgumentError, _("Attempt to assign a value to unknown setting %{name}") % { name: name.inspect }
          
          ArgumentError:
            Attempt to assign a value to unknown setting :trusted_server_facts
          # ./vendor/bundle/ruby/2.4.0/bundler/gems/puppet-1fe4d9584e03/lib/puppet/settings.rb:1405:in `set'
          # ./vendor/bundle/ruby/2.4.0/bundler/gems/puppet-1fe4d9584e03/lib/puppet/settings.rb:339:in `block in initialize_app_defaults'
          # ./vendor/bundle/ruby/2.4.0/bundler/gems/puppet-1fe4d9584e03/lib/puppet/settings.rb:335:in `each'
          # ./vendor/bundle/ruby/2.4.0/bundler/gems/puppet-1fe4d9584e03/lib/puppet/settings.rb:335:in `initialize_app_defaults'
          # ./vendor/bundle/ruby/2.4.0/gems/rspec-puppet-2.6.14/lib/rspec-puppet/adapters.rb:53:in `setup_puppet'
          # ./vendor/bundle/ruby/2.4.0/gems/rspec-puppet-2.6.14/lib/rspec-puppet/adapters.rb:112:in `setup_puppet'
          # ./vendor/bundle/ruby/2.4.0/gems/rspec-puppet-2.6.14/lib/rspec-puppet.rb:114:in `block (2 levels) in <top (required)>'

Probably due to puppetlabs/puppet#6979 and https://tickets.puppetlabs.com/browse/PUP-6112

See https://travis-ci.org/puppetlabs/puppet-resource_api/jobs/415084960 for the entire failed run.