Enhancement: bugresolve should optionally set the milestone

Some launchpad workflows don't pre-set the milestone, instead collecting all fixes on a series. For this workflow, it would be helpful if tarmac would fill in the "correct" milestone, using something like the following logic:

1) nearest milestone with target date in the future
2) if no milestones are in the future, sort remaining open milestones lexically by name and pick the first
3) If all milestones have dates and we are past all dates, pick the newest of those.
4) override all that logic with a config setting of "default_milestone" which is the specific milestone name to use (which will even select a non-active milestone).