Add replication key for gerrit from hiera

The ssh key in ~gerrit2/.ssh/id_rsa which is what is used for outbound
ssh-based replication is currently just kinda there by hand. Add management
of the files there.

Change-Id: I5bfea4543d6eb46ba2e9f3c791f4e6b6c5534522
Closes-Bug: 1209464
Merge "Add replication key for gerrit from hiera"