Migrate GitLab Container Registry storage to Amazon S3

The current plan is to migrate the Container Registry hosting away from our current local file storage system onto Amazon S3.

Pre-relocation questions to answer:

• Bucket Name?
• AWS S3 Zone?
• Configure CloudFront?

Migration plan:

• Configure S3 and IAM: https://github.com/docker/distribution/blob/master/docs/storage-drivers/s3.md#s3-permission-scopes
• Use CloudFront in front of S3 backend: https://github.com/docker/distribution/blob/master/docs/storage-drivers/s3.md#cloudfront-as-middleware-with-s3-backend
• Allow to configure different file storage for S3: gitlab-org/omnibus-gitlab!816 (merged)
• Improve GitLab to support external image storage: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4677
• Synchronize content of /var/opt/gitlab/gitlab-rails/shared/registry filesystem to external storage
• Declare Cut Over Date
• Do final synchronization to external storage
• Switch configuration: https://github.com/docker/distribution/blob/master/docs/storage-drivers/s3.md#cloudfront-as-middleware-with-s3-backend

/cc: @ayufan @tmaczukin @pcarranza @stanhu