nabu-config: remove --bootstrap
nabu-config was foreseen to work in three modes:
- "bootstrap": create a template configuration file from scratch
- "convert" : convert a PyHST2 ".par" file into a nabu ".conf" file
- "validate" a nabu configuration file
Only (1) is used. Mode (3) is implicitly used when running
nabu nabu.conf, and (2) is cumbersome and not foreseen.
--bootstrap parameter should be deprecated and be the only working mode.