Load custom user manifests from puppet/manifests/manifests.d/*

This change configures Puppet to import manifests.d/*.pp into site.pp, allowing
users to have custom Puppet fragments and have them be ignored by Git. It is
meant for advanced users for whom Roles.yaml is not sufficient.

This setup mirrors the setup for loading MediaWiki configuration snippets in
settings.d.

Change-Id: I7f6e96cb1080dae6164e6065253717c0af850ed2
Merge "Load custom user manifests from puppet/manifests/manifests.d/*"