replace `cvmfs_server lstags` by `list-tags`
add backward compatibility alias lstags -> list-tags
adapt bash completion lstags -> list-tags
adapt lstags usages in cvmfs_server
adapt integration test 520
FIX: typo
Merge pull request #438 from reneme/fix-rename_lstags

FIX: Rename cvmfs_server lstags to list-tags for consistency