[COOK-3756] Change postgresql.conf mode from 0600 to 0644

0600 break "pg_wrapper".

If the user isn't root or postres, it can't check if cluster is created and fail like:

```
# sudo -u user pg_dump --version
Error: /etc/postgresql-common/user_clusters line 23: cluster 9.1/main does not exist
```

https://tickets.opscode.com/browse/COOK-3756