Add support for credentials file (WIP)

[radeksimko]

This is adding support for new config option credential_file as discussed earlier in #151 and #15 (comment)

I tried to implement it exactly the way @mitchellh described in #15 (comment) and it basically works except that I probably don't fully understand the lifecycle - when finalize! & validate gets called, which is why the validation is broken and always assumes conflict.

Maybe I can move the whole block in finalize somewhere else, so it gets executed after validate?

Also there's one more config option that can be added, profile defaulting to ENV['AWS_PROFILE']:
https://github.com/aws/aws-sdk-ruby/blob/master/spec/aws/core/credential_providers_spec.rb#L395

Here's an example of AWS_PROFILE variable name use (just FYI, that I'm not reinventing the wheel here):
https://github.com/aws/aws-sdk-core-ruby/blob/90f34bcb002ef8be418502164063fe297727f9d5/aws-sdk-core/lib/aws-sdk-core/shared_credentials.rb#L13-14

Here's an example of how the generated ini file may look like:
https://github.com/aws/aws-cli/blob/master/README.rst#getting-started

[kenorb]

This needs to be rebased if it's still relevant.