add legacy migration switch to cvmfs_server import
cosmetics in cvmfs_server usage
error message clearification
minor refactoring in cvmfs_server import
legacy catalog migration is now opt-in
adapt test case 519-importlegacyrepo
make sure repository apache config file is clear before creation
TEST: recreate a repo spool area from storage and keys
FIX: cannot overwrite keys at standard location
Merge pull request #319 from reneme/data_import

Allow For Recreation of the Repository Spool Area Given the Storage and Keys

This extends cvmfs_server import to work with non-legacy repository. Essentially it creates a fresh spool area for a given repository backend storage location and the matching keychain. The code is basically identical, except that catalog migration from 2.0.x is now an opt-in.
Additionally I provided a test case to verify the functionality.

This implements a feature request by Dmitry Ozerov.