Rewrite myhostname in /etc/postfix/main.cf in the install script

This is necessary because the install script is also run on creating a
new EC2 instance from the AMI in order to update the configuration with
the new internal and external names; the hostname will be different in
this case and otherwise myhostname wouldn't be rewritten.
Merge branch 'feature/install-script-rewrite-hostname' into rails-3-develop