Automatically set proper pg_host

For a setup with multiple release nodes database.yml should contain the IP of the database server (not string 'localhost').
Note that you can have 1 release_role on host X and a db role on host Y. This line would set the default `pg_host` to `'localhost'`. A simple fix would be: 

`roles(:all).count > 1 ? primary(:db).hostname : 'localhost'`

but this is fairly crude, because this is also triggered if you have db and app running on the same node, but a non-release web on a different one. A better solution is to check whether there is 1 release_role and that its hostname is equal to primary(:db).hostname. @bruno-  What do you think?