increase number of images returned by catalog

[tomzo]

V2 registry uses pagination on the _catalog endpoint. By default it results ~50 repositories. It can be controlled by query parameter n. Such as docker-registry..com/v2/_catalog?n=1000. In the existing code this seems ignored at all and the list of all images is always shorter than actual when there are lots of them. This should be probably some configuration value. So far I have added n to the HTTP request and increased default to 500.