allow to specify shell to use

[nhoening]

Pythons subprocess module uses /bin/sh as default.

I tried for some time already, but it is not easy to make it use a bash session. Here is an idea: http://www.saltycrane.com/blog/2011/04/how-use-bash-shell-python-subprocess-instead-binsh/

What I wanted was to have bash use its config settings from .bashrc, therefore we need to pass the -i option to /bin/bash.