FIX: Symlinked Data Storage Wasn't Handled Correctly

This fixes the bug in `cvmfs_server` that was found by Dimitry Ozerov yesterday. It prevented `cvmfs_server mkfs` from properly creating a new repository when the repository's backend storage location (*/srv/cvmfs/\<fqrn>*) was a symlink to somewhere else.
Additionally this contains a regression test and an integration test for the stratum 1 case.