I used DHCP for my cluster. The ambari server ip could change. I tried to play with postgresql and haven't figured out if it works or not. Postgresql does have information about the ip and hostname. I will let you know once I figure out a solution.
And this scenario is not fully tested and supported. Also when you update configuration files directly from the server, the changes will not take effect. The risk is that there might be potential issues that we couldn't find out at early stage.