Allow knife to run using IAM role

Use case: C/D server running in EC2 with IAM role assigned and no on
machine creds.

This was a patch I wrote in a hurry. Logic revision suggestions are
very welcome.
Call locate_config_value over Chef::Config

I’ve only changed this in conditionals I added as a part of setting the
:use_iam_profile attr
Add unit tests for use_iam_profile

Test default value of use_iam_profile == nil
Test a connection is created without access keys while use_iam_profile
== true
Rename tests to better describe what they do
Simplify use_iam_profile configuration
Documentation update for IAM role credential feature
Merge pull request #242 from opscode/adamed/use-iam-profile

Allow knife-ec2 to run using IAM role