doc: fix documentation build.
remove: dnf.const.LOG_TRANSACTION no longer needed.
logging: do not do logging.basicConfig() from CLI.

it's a trap.
put the name of the loglevel into the logfiles
logging: replace setup_from_dnf_levels with a helpers that takes the conf object.

It's less clean but a lot neater. Plus with syslog there's going to be more
values to be passed from the conf. The logging.setup method stays pure.