Lower max connetions to 20 in acceptance tests

The max memory of postgresql is partly determined by the max
connections. Since we don't actually care about the value here, we set
it to something lower. 21 should be non-default.

Author: ekohl

spec/acceptance/overridden_settings_spec.rb

@@ -12,7 +12,7 @@ class { 'postgresql::server':
         },
       },
       config_entries => {
-        max_connections => 200,
+        max_connections => 21,
       },
       pg_hba_rules   => {
         'from_remote_host' => {