Allow setting private IP to ENI

When setting private_ip as input to the EC2 instance, an error is thrown. It is not possible to set an aws_network_interface to the resource aws_intance and asking for a private_ip as show:

Error: Conflicting configuration arguments
│
│   with module.ec2_nifi_aero["nifiaero-node3"].aws_instance.ec2_instance,
│   on .terraform\modules\ec2_nifi_aero\main.tf line 10, in resource "aws_instance" "ec2_instance":
│   10: resource "aws_instance" "ec2_instance" {
│
│ "network_interface": conflicts with private_ip

This MR should allow to use and set a private_ip to the ENI and so the EC2 instance.