s3 driver fails to accept AWS credentials supplied via environment

[Shrishail Kariyappanavar]

As of now the s3 driver only accepts credentials to AWS via glance-api.conf. We need to relax that and allow users to supply credentials via environment as well.

For example, aws credentials helper runs a server to fetch short lived credentials via an HTTP server, the server endpoint can be added to env via AWS_EC2_METADATA_SERVICE_ENDPOINT variable.

The boto library being used already supports that, we just need to provide an option to initiate boto session without any parameters for cases where access_key and secret_key are not configured in glance_api.conf