cvmfs_server Runs a Health Check Before State Transitions

This adds a health check before doing *resign*, *check*, *transaction*, *publish*, *rollback* or *migrate* in `cvmfs_server`. Currently the health check only makes sure that the mount points at `/cvmfs/<fqrn>` and `/var/spool/cvmfs/<fqrn>/rdonly` are properly configured. It is subject for extension of course.
Furthermore it is still not possible to recover from a detected malformed state using the server tools.

Additionally there is an integration test that provokes all of the checked situations.