Set shell to bash when adding users

[anandology]

When the users are added to the jupyterjub, the shell defaults to sh instead of bash.

For example, the following are the entries in /etc/passwd file in the jupyterhub container.

kaustubh1:x:1000:1000::/home/kaustubh1:/bin/sh
kaustubh2:x:1001:1001::/home/kaustubh2:/bin/sh
anand:x:1002:1002::/home/anand:/bin/sh

This leads to starting sh when a terminal is opened from jupyterlab.

I think we need to fix this by explicitly specifying the shell when adding users.

[anandology]

Fixed it in current instance by running:

tail -15 /etc/passwd | sed 's/:.*//' | xargs -n1 echo chsh -s /bin/bash

[nikochiko]

This will need to be added in jupyterhub-simple image instead.
https://github.com/pipalacademy/jupyterhub-simple/blob/8f13553db638646ecefe8ce5909f92795be171d6/src/add-users.py#L14