Match the ensure value of the manifest

This was changed in 68d8c64 to fix
puppet lint issues, but as the test suite was not run it passed through.

spec/unit/classes/server_spec.rb

@@ -202,7 +202,7 @@
           it 'contains file private.pk8' do
             is_expected.to contain_file('/etc/puppetlabs/puppetdb/ssl/private.pk8')
               .with(
-                ensure: 'present',
+                ensure: 'file',
                 owner: 'puppetdb',
                 group: 'puppetdb',
                 mode: '0600',