Add missing ml2 plugin to migration 1fcfc149aca4

Migration 1fcfc149aca4_agents_unique_by_type_and_host
shouldbe applied to ml2 plugin as well.

Change-Id: I3f5a2fcfec49316fbe06d6221d003aeb2599bca7
Closes-Bug: #1350326
Merge "Add missing ml2 plugin to migration 1fcfc149aca4"