Not sure that it's actual for you since question asked 5 months ago, but you need to disable local pgsql service:
postgresql['enable'] = false
And set connection options:
postgresql['vip'] = '<IP ADDRESS or HOSTNAME of DB server>' postgresql['port'] = PGSQL_PORT postgresql['sql_user'] = '<USERNAME>' postgresql['sql_password'] = '<PASSWORD>'
And finally run reconfiguration to generate new configuration files and restart necessary services:
chef-server-ctl reconfigure
Don't forget to import DB dump to your RDS instance, I'm not sure that chef will create all the things in this case.