Feature: Centralise the Use of `sudo` in `cvmfs_server`

This introduces the function `get_user_shell()` that returns a shell invocation to impersonate the repository owner of a provided repository in `cvmfs_server`. With that patch `cvmfs_server` has only *one* occurrence of `sudo`, making it easily replaceable (i.e. [CVM-245](https://sft.its.cern.ch/jira/browse/CVM-245)).