Set up postgres secondary

- Factor out postgres stuff into it's own module
- Set up streaming binary replication

Note that this does not cover getting the primary's database onto the
secondary and getting it up to speed.
Live clone of postgres database

This introduces a script that can be run on a new secondary to bring it
up to speed with a primary without having to take the primary down.

On the priamry the checkpoint_segments and wal_keep_segments have been
increased to allow the secondary to catch up on writes that happen
during it copying the data over.

A login user has been introduced on the primary as well, as that is
needed for the pg_basebackup command.

This is modified slightly from
http://www.rassoc.com/gregr/weblog/2013/02/16/zero-to-postgresql-streaming-replication-in-10-mins/

I've gone with syncing to a separate directory and then rsyncing to
preserve things like the certificate and recovery.conf

This includes some fixes that @timmow noticed around rsync.
Set pp_postgres::server in primary/secondary def

The classes depend on pp_postgres::server so it should be explicit in
their definition.
Factor out a pp specific hba rule define

This wraps up some common defaults for the performance platform to both
reduce duplication and avoid the risk of bad configurations.
Remove redundant postgres code from performanceplatform

It has moved into the pp_postgres module.
Remove redundant comment
Add link to PostgreSQL docs for HBA conf
Merge pull request #555 from alphagov/feature/postgres-replication

PostgreSQL streaming replication