cli/request: support passing instance parameters from a file

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 also teach slapos.core about the instance schema, which makes it possible to
understand the serialisation, so that we no longer need to use `_ = json.dump(params)`
to encode parameters.

This also emits a warning when requesting with parameters which do not match the schema.


See merge request !215