cvmfs_server tag -a does not work on a publisher backed by a gateway

[vvolkl]

Instead, it fails with

[ERROR] Session token file has to be provided when upstream type is gw.
failed to init environment

The cvmfs_server tag command has no option to pass a session token to the underlying swissknife command - instead this has to be executed on the gateway directly. I don't see a reason why this has to be this way - at least it should be documented in https://cvmfs.readthedocs.io/en/stable/cpt-repo.html#managing-existing-named-snapshots