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
(cherry picked from commit 30f1a75e790a512ea8b2b4997a634a50070fef9f)
Merge "Add missing ml2 plugin to migration 1fcfc149aca4" into stable/icehouse