nabu-config: remove --bootstrap

nabu-config was foreseen to work in three modes:

1. "bootstrap": create a template configuration file from scratch
2. "convert" : convert a PyHST2 ".par" file into a nabu ".conf" file
3. "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.

The --bootstrap parameter should be deprecated and be the only working mode.