Allow S3 snapshot provider to work in non AWS environments

[faraazkhan]

Currently the S3 snapshot provider requires the presence of the EC2 metadata service, limiting its use to only instances deployed within AWS. Many cloud providers provide S3 compatible APIs, and it may be possible to allow the use of the S3 snapshot provider to send snapshot to GCS for example (https://cloud.google.com/storage/docs/interoperability).

Another use case is people running in hybrid cloud environments where while the etcd cluster runs on Openstack or bare metal but an S3 compatible API is available for backups etc.