Add ability to define persona-type credentials file within boto.cfg

This expands upon the existing feature to set the environmental variable AWS_CREDENTIAL_FILE to a user-defined file with multiple personae. In certain work environments that manage configuration files - but not environmental variables - centrally, it would be useful to set this within a place like boto.cfg.

In the [Credentials] section of a boto.cfg (or similar) we now support an option called aws_credential_file, which represents a path to a file containing at least one persona. This does NOT override the extant behavior of setting a keypair in [Credentials] - it only kicks in if aws_access_key_id and aws_secret_access_key are missing.