Allow user to mark host as evacuate before removing host

[xyuanlu]

Is your feature request related to a problem? Please describe. Right now Helix does not provide user with an option to prepare planned host removal. Many replicas will have n-1 replicas during host removal and adds risk for availability loss.

Describe the solution you'd like Helix could provide a feature that allow user to tag to be removed host and proactively move replicas to other hosts.