Default to use `/bin/sh`

There is no guarantee that `bash(1)` is installed on a system and if it
is, it can be in another location than `/bin/bash`.  If `bash(1)` has to be
installed, it should be looked after using:
~~~
/usr/bin/env bash
~~~
Otherwise, it is possible to use `/bin/sh` (which is often `bash(1)` on a
GNU/Linux system) instead of `/bin/bash`.  This is what this patch does.
It also fix the regression tests accordingly and update them to use
`bash(1)` (looking for it using `env(1)`) instead of `sh(1)` in the test
overriding job_template.

This way, whenever can be used out of the box on systems installing
`bash(1)` in other locations than `/bin`.