AWS integration work

I’ve got this to a working PoC state. Some changes to BinderHub code are needed and some documentation on how to properly configure ECR and CodeCommit. I’ve created https://github.com/jupyterhub/binderhub/issues/918 and updated https://github.com/jupyterhub/binderhub/issues/705 to point the changes that we need to discuss about BinderHub. Once we decide the paths I’ll send PRs for the necessary code and docs changes.