Allow scc check-milestone to correct wrongly assigned milestones

Previously, scc check-milestone was skipping the milestone edition if any
milestone was assigned to a PR. With this commit, the title of the milestone
(assumed to be unique) is additionally checked and a new milestone is
reassigned if applicable.
Merge pull request #191 from sbesson/check_milestone_set

Allow scc check-milestone to correct wrongly assigned milestones