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

This also adds missing tests for slapos request command