Rudy/hot standby

@miketheman 
Managed to have it working.
On a replication-node, I can
- Do a wal-e backup-push --hot-standby
- rm the $PG_DATA folder
- Do a wal-e backup-fetch
- Add a recovery.conf file with a restore-command containing 'wal-e wal-fetch'
- start postgres

and it will come up and follow the master


This will only work if you do it on a real hot-standby instance. 
If the backup is done this way on a host that is running as a master (added a check to prevent it), you are not going to be able to restore cleanly.