ELB(s) cause a weird formatting behaviour - it appears as EL**s**
Added choices documentation to the state option

Moved `AWS_REGIONS` into `ec2` module
Created `ec2_connect` method in `ec2` module
Updated modules able to use `ec2_connect` and `AWS_REGIONS`

The `ec2_ami`, `ec2_elb`, `ec2_tag`, `ec2_vpc`, `route53`, and `s3` modules
all canonicalize the AWS access and secret key params as
`aws_access_key` and `aws_secret_key`. However, following the fixes for #4540,
those modules now use `get_ec2_creds` from `lib/ansible/module_utils/ec2.py`,
which requires access/secret key params to be canonicalized as
`ec2_access_key` and `ec2_secret_key`. As a result, AWS credentials passed
to those six modules as parameters are ignored (they instead always use
the AWS credentials specified via environment variables, or nothing).

So this change fixes those six modules to canonicalize the
AWS access and secret key params as `ec2_access_key` and `ec2_secret_key`,
allowing them to again accept AWS credentials passed via module params.

No idea if I'm rebasing properly or not.  This is my first attempt.

Boto blindly assumes the us-east-1 region if you don't hardcode a
region in it's config, so you could end up attempting to modify ELB's
in one region from a totally different region.  If a region isn't
specified then default to the region that the module is being run
within rather than the default us-east-1 region since it's a pretty
safe assumption that you intend to work on the ELB's within your
current region.

Also throw an error if a specified ELB instance doesn't exist.  The old
behavior would be to silently succeed with changed=false, so if you had
so much as a typo in the name of your ELB (or were in the wrong region
like my initial testing) you wouldn't get a clear indication that a
problem had occurred.

Revert "Added an elb_region parameter for interacting with the ec2 elastic load balancer to work with regions other than us-east-1"

This reverts commit 1264a5b4.

Added an elb_region parameter for interacting with the ec2 elastic load balancer to work with regions other than us-east-1

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.