Deprecate 'config.aws_region' option and add 'config.default_aws_region' option.

[simnalamburt]

config.aws_region overrides AWS_REGION environment variable, and changing config variables in ~/.ecl/config.rb is not ergonomically convenient. So many users just comment out the config.aws_region variable and use the use the AWS_REGION.

So let's add new configuration option named config.default_aws_region which does not overrides AWS_REGION environment variable, and deprecate config.aws_region option.

TODOs

• Remove config.aws_region option from documentation and eclare config template.
• Print a warning to the STDERR when config.aws_region variable is set by the user.
• Implement new config.default_aws_region option and mention it on docs and config template file.

How do you think?

^{(Blocked by #9)}