Core: new workflow with users preferred shells

- U:D:preferredShell renamed to U:D:preferredShells
  - user can specify multiple shells which he prefers ordered by
    preference
- new module for attribute UF:V:shell
  - get shell for user on facility based on users preferences
- basic test of UF:V:shell module
Merge new workflow with users preferred shells

Based on pull request #29 from papperwing/shells
Clean up of changes in code.
Renamed module U:D:preferredShell to U:D:preferredShells.