toolcontext: allow user to manually set `nit_version`.

Useful for testing.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
template: add `Template::addn` service

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
toolcontext: add `bash_completion` generation option for nit tools.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
tests: add `test_toolcontext`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>