let systemuser l10nupdate have bash as shell so it can run
l10nupdate scripts via dsh on new mw* hosts.

< apergos> sudo -u l10nupdate ...
< mutante> if i'd give it a bash, it works

< anomie> apergos, mutante: Yeah, it used to work, until relatively recently.

< anomie> mutante, apergos: So, since the /bin/false shell is the problem,
          can we get an appropriate shell set?
          Or is there some other way for sync-l10nupdate-1 to work?

Bug: 50433
Change-Id: I55cbb6177fd30bf21bfa1ebc37f88a9eeca8c98c