FIX: `rmfs` didn't Cleanup Backend Storage + Proceedure Cleanups

This mainly fixes a problem introduced yesterday, namely that `cvmfs_server rmfs` failed to properly remove the repository's backend storage. This lead to failing test cases in the cloud tests.
Additionally this does some minor restructuring of the creation/import and deletion of repositories. Furthermore it cleans up the logging of `cvmfs_server mkfs` and `cvmfs_server rmfs`, which before happened partially in the main functions and in utility functions.