Add option to limit number of external server URLs tried

Our list of external server urls for osgstorage.org repositories is getting very long, like on the order of a dozen, and they are all caching servers on the same origin server. I am concerned that under error conditions clients could end up trying a large number of them, some of them very far away geographically, and that it would be better to limit the client to use a smaller number, the first N ordered by geo IP. This would also enable us to use one list including caches all over the world. Maybe something like CVMFS_EXTERNAL_MAX_SERVERS. At the same time we probably might as well do the same thing for regular server urls, CVMFS_MAX_SERVERS.