Decouple log level from developer mode

I find annoying that the developer mode change the logging level.
I think it will be better to just link the log level to the number of verbose flags.

assigned to @ced

added trytond type::feature + 1 deleted label

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review66521002 updated at https://codereview.tryton.org/66521002/#ps1

New changeset 43d0e1e1f44b by Cédric Krier in branch 'default':
Use number of verbose flag as log level
https://hg.tryton.org/trytond/rev/43d0e1e1f44b

added 1 deleted label and removed 1 deleted label

closed

New changeset e82a43b7d254 by Cédric Krier in branch 'default':
Use number of verbose flag as log level
https://hg.tryton.org/tryton-env/rev/e82a43b7d254