Support named AWS profiles

[John W. Lamb]

The AWS CLI config file format allows specifying multiple sets of account credentials in the same file by placing them in unique INI sections: <https://docs.aws.amazon.com/cli/latest/userguide/cli-multiple-profiles.html>

The Boto library supports use of these named profiles via the boto3.session.Session(..., profile_name="") named parameter: <http://boto3.readthedocs.io/en/latest/reference/core/session.html>

There doesn't seem to be a means of providing the profile name for s3/s3+http backup/restore in duplicity, but there should be. I would prefer this capability versus various environment variable based options.