Added support for config parameters elastic_ip => '<ip>', allocate_elast...

We needed support to add elastic ips to instances on AWS. I've added a couple of config params:

1. elastic_ip : 'ip' - user explicitly specifies the elastic ip to be associated with the instance
2. allocate_elastic_ip  : 'standard/vpc' -  since these are the 2 options. This allocates a new IP and associates it with the instance.

Please let me know your thoughts. I am ready to modify these to get my pull request accepted by such an awesome project! 
Thanks