Allow security tokens and profiles to be used as arguments
to the 'common' ec2 modules

Mostly refactoring to provide two new methods,
`get_aws_connection_info`, which results in a dict that can be
passed through to the boto `connect_to_region` calls, and
`connect_to_aws` that can pass that dict through to the
`connect_to_region` method of the appropriate module.

Tidied up some variable names

Works around boto/boto#2100

profiles don't work with boto < 2.24, but this detects for that
and fails with an appropriate message. It is designed to work
if profile is not passed but boto < 2.24 is installed.

Modifications to allow empty aws auth variables to be passed
(this is useful if wanting to have the keys as an optional
parameter in ec2 calls - if set, use this value, if not set,
use boto config or env variables)

Reworked validate_certs improvements to work with refactoring

Added documentation for profile and security_token to affected modules