Add legacy to the possible server group policies

To make the legacy GroupAffinityFilter and GroupAntiAffinityFilter works
the scheduler code needs to understand the 'legacy' policy in the server
group.
This is a stable only fix as in master the related code path has been removed
after deprecation.

Closes-Bug: #1394551
Change-Id: Ib46e035fa2687074ca05a902aa6321c507d01abc
Merge "Add legacy to the possible server group policies" into stable/juno