Manage ssh config in separate pieces

In order to add new ssh config for another team, I was looking at pasting things into the "extra" field of gds-ssh-config. This felt icky.

This PR introduces puppetlabs-concat to manage ssh config in separate pieces. This means that each team can manage their own ssh config separately, and furthermore users can add any extra config for themselves in their own manifest.

cc @alexmuller @bradleywright @robyoung, all of whose manifests are affected by this change.