Adding module to de-register/register EC2 instances

This module handles AWS EC2 ELB registration.

* De-registration requires the instance id which can be looked up using
  the ec2_facts module.
* Registration requires both the instance id and the elbs that the
  instance belongs to.

De-registration will return an ec2_elb fact that can be used for
registration in post_tasks.
marking the state changed only when there are ELBs to work on
doc update
changing arg name for consistency, absent/present for registration
Parameter and variable naming issues
Merge pull request #2815 from jarv/devel

EC2 ELB management