proxy request_node_count set less than ec_ndata prevents getting ec objects

[Alistair Coles]

If the proxy_server request_node_count option is set to less that ec_ndata then the proxy won't issue enough back end requests to rebuild an EC object, so GETs will always fail.

The preferred way to set request_node_count is to us 'N * replicas' but it is allowed to use an int value, and that value is not constrained by the EC policy parameters. A likely fix is to force request_node_count to be at least ec_ndata for an ec policy.