[devops] Implement new workflow for maintenance milestones

[Mike Scherbakov]

Current workflow is complicated for developers and has some other drawbacks. Thus, in particular:
* when I'm developer, I propose change to master and I think I would need to propose it to 4.1A. Currently we say somewhere that you would need to apply tag "backports-4.1.1"... and after it's being committed, change milestone to 4.1.1
* when you changed milestone to 4.1.1, it's hard to guess that this bug was actually fixed in 5.0. So it gets complicated when you create release notes and provide statistics for release.

Take a look at some other workflows, can we target bug somehow to multiple milestones/series? Take a look at https://launchpad.net/neutron/+series as one of the examples. How do they use "future" series, and "next" milestone? May be we can reuse same approach?

Please share your research, and after agreement, we will implement it.

Thanks!

[Aleksandra Fedorova (bookwar)]

https://wiki.openstack.org/wiki/Fuel/Launchpad_Series