Jupyter: Switch to using JSON in instance parameters

The reason is: we are going to integrate Jupyter into ERP5 SR, and in
ERP5 SR code, there is already established infrastructure, which
requests slave instances. That infrastructure passes parameters in
serialized (= json) form, that's why we need to switch.

On the other hand, slapos.cookbook:slapconfiguration.serialised handles
non-json instance parameters as well: when there parameters format is
not

    {'_' -> <json>}

it decodes them as slapos.cookbook:slapconfiguration would do:

    https://lab.nexedi.com/nexedi/slapos/blob/69229988/slapos/recipe/slapconfiguration.py#L232
    https://lab.nexedi.com/nexedi/slapos/blob/69229988/slapos/recipe/librecipe/__init__.py#L51

So from this point of view, though we don't really need it here (Jupyter
SR is relatively new), we are staying backward compatible.

/cc @tiwariayush