Add stratum 1 API for dynamic ordering of stratum 1s

As agreed in the "CernVM-FS Key and Config Distribution Meeting" of 28 March, add a REST API for stratum 1s at

  http://${STRATUM1-HOST}/cvmfs/$fqrn/api/v1.0/geo/<proxy address as seen by cvmfs client>/<alphabetical list of stratums separated by comma>

that returns an ordered list of indexes of the input stratums.