Make CI waiting time configurable (+ option refactoring)

At the moment, it was hard-coded that marge would wait 15 minutes for CI to finish. Different teams have different needs, so now this parameter can be set with --max-ci-time-in-minutes, which defaults to 15.

In doing so, this PR also refactors the handling of configuration parameters, so that we don't need to keep an ever-growing list of arguments in the Bot constructor. In the process, we remove the dependency of MergeJob on Bot and modify the --embargo parameter so that it can now receive all the embargo times separated by commas.