Support a syntax like:

      slapos request \
        --node=computer_guid=local \
        --parameters-file=~/request.json \
        ERP5 \
        https://lab.nexedi.com/nexedi/slapos/raw/1.0.145/software/erp5/software.cfg

to request an instance with parameters in ~/request.json file.

This automatically understands the serialisation, when software uses json-in-xml
serialisation, no need to use `_` argument with the encoded json.