lib: reduce impact of bad import cycle

Follow-up to e0f31c7d0f5e.

It is unfortunate that generic lib code depends on
kallithea.lib.paster_commands.common - it would make more sense if it was the
other way around. We thus move the import down to inside the function that
needs it.

The import of kallithea.lib.paster_commands.common in db_manage triggered its
import of logging.config . middleware.py thus also got logging.config when it
just imported logging and things worked. Without this import of
paster_commands, we also have to fix the import in db_manage.