Fix container-server sorting for manifest listings.

The current code will always contact the container server for the
first replica of the container; this fixes it to use the same
backend-sort as all the other places in the proxy that talk to
backends.

Change-Id: If1c168b95bfc5cd120036869f3066549b7a1000a
Update documentation for account_quota middleware

Change-Id: I3dc7930ee2a1eb26b1f60e83fce2bc1bea0a8b0f
Merge "Fix container-server sorting for manifest listings."
Merge "Update documentation for account_quota middleware"