plainbox:impl:unit:job.py: Added partial_category_id property

To be used without qualified id in exporters.
plainbox:impl:exporter: Add partial_category_id to the result_map jobs
plainbox:impl:exporter:xlsx.py: Add support for the new units categories

The way we build the job tree based on categories can now use the ones
provided by the job themselves.
The Legacy way based on local jobs can still be used but the new mode takes
precedence over the legacy one.
plainbox:impl:exporter:test_init.py: Update after category_id creation
"automatic merge of lp:~sylvain-pineau/checkbox/xlsx_exporter_new_categories/ by tarmac [r=sylvain-pineau][bug=1397280][author=sylvain-pineau]"