Full Data Integrity Check as Opt-In in `cvmfs_server check`

This adds a `-i` option to `cvmfs_server check` to enable the full data integrity check provided by `cvmfs_swissknife scrub`. By default this is disabled.

Additionally the integration test cases run this check now be default.

**Note:** This is dependent on another [Pull Request](https://github.com/cvmfs/cvmfs/pull/350)