Add cvmfs client use of API to order stratum 1s

Implement client's use of the API developed for https://sft.its.cern.ch/jira/browse/CVM-629. This issue depends on that one being finished.

The relevant notes from the meeting are:

• Automatic ordering can be turned on and off through a configuration parameter. The cvmfs default is off (which might change in the future), the cvmfs-cern-config default will be on.
• The client should show the effective ordered list through an extended attribute