Don't specify port for postgresQL provider

Don't specify port, as the RDS instance doesn't exist yet in the
moment of postgresQL provider initialization, which then breaks,
because port is returned as empty quotes, which doesn't convert to
string. See hashicorp/terraform#2430

terraform/cloudfoundry/cf_dbs.tf

@@ -1,6 +1,5 @@
 provider "postgresql" {
   host = "${aws_db_instance.cf.address}"
-  port = "${aws_db_instance.cf.port}"
   username = "${aws_db_instance.cf.username}"
   password = "${aws_db_instance.cf.password}"
 }