Fixing bug where if both private_ip and assign_public_ip in ec2 are set ansible fails out

Ansible fails out.

This fixes this bug:

https://github.com/ansible/ansible/issues/6987


I tested all 3 variations after creating this patch: Private_ip alone, assign_public_ip and both set. All work. It also passes the "make tests".

This is my first patch so let me know if I did something wrong.