TEST: Integration Test 519 is now Self-Contained

This changes the integration test case 519 to become self-contained by reusing code and repository tarballs from test case 550. Before test 519 downloaded a snapshot of the Boss repository [1] which is now already migrated to 2.1 and cannot be used as a guinea pig anymore.

[1] http://cvmfs-stratum-zero.cern.ch/opt/boss