Make keysum return None if not key_check (this case wasn't covered).

timeit shows a speedup of ~3x on Python 2.7.5 x86_64.
It also makes the code a bit shorter.

Change-Id: I9b89d433b545269d111b3c290b6411aabf58dd24

Change standard AWS access and secret key parameter names to aws_access_key and aws_secret_key. Fixes an authentication problem with the rds module and standardizes the naming convention on something more encompassing than ec2_*_key.

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.

Check for Walrus endpoint by looking to see if the endpoint is not AWS.

This fixes a bug where the user specifies an AWS endpoint via S3_URL,
but the code interprets it as Walrus and then fails.

Fixes #4458

dest in s3 module does not work with ~username notation for home directories.  Wrapped dest definition in os.path.expanduser() which will return the expanded path in case of successful tilde expansion and otherwise the original value

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

Adds some additional docs about the module parameters for s3
and ec2 modules.

  Migrated all examples: in DOCUMENTATION=''' string to standalone EXAMPLES=''' string
  Added deprecation warning to moduledev.rst and remove deprecated example from it
  Fixed up a few typos and uppercased some acronyms.
  add consistency to how EXAMPLES are formatted

to indicate that remote file should be overwritten if a file with the
same filename exists.