[devops] Passed milestones are not disabled for assigning bugs

[Mike Scherbakov]

I've discovered that
"Create release and deactivate released milestone on LP for both Fuel and MOS projects"
in 5.1 release checklist whether was not fully done or there is some issue with LP. I can see a few bugs targeted to 5.1 milestone:
https://bugs.launchpad.net/fuel/+bug/1371471
https://bugs.launchpad.net/fuel/+bug/1356965
and there are a few more.

Looks like I can still assign bugs to 5.1, while I should not be able to (like I can't for 4.1). Please fix and ensure we don't have this issue with 5.1.1 / 6.0.

[Aleksandra Fedorova (bookwar)]

Released milestones, like 5.1, are disabled, but they are not removed from old bugs. If someone changes bug status from Fix Released to Confirmed (as in https://bugs.launchpad.net/fuel/+bug/1356965), the bug will stay with the old milestone unless it is changed.