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.*