Vagrant env for Postgres replication labs.

This sets up three VMs: "alpha", "beta", and "delta" ("gamma" got lost on the
way to market). These come up with PostgreSQL 9.2 running, ready to be
configured as a replication group. I've also included some scripts (in
/vagrant/scripts/ on the VMs, in . on the host) that allow rapid replication
changes.

To configure the VMs, you _must_ run the `mk-keys` script before running
`vagrant up`, to create the initial SSH host keys and the `postgres` user's
SSH key. (If you forget, that's okay. Run the scripts, then run `vagrant
reload`.)

You should also set up `/etc/hosts` on your host system to give the three VMs
notional hostnames. The `Vagrantfile` includes configuration to tell the VMs
to use the host's resolver, so this will also allow you to use these names in
the guests. The following hosts entries are sufficient:

    192.168.50.20    alpha
    192.168.50.21    beta
    192.168.50.22    delta