Sync db.models.Instance* with migrations.

This patch syncs models with migrations for:
    - InstanceIdMapping
    - InstanceMetadata
    - InstanceSystemMetadata
    - InstanceTypeExtraSpecs
    - InstanceTypeProjects
    - InstanceTypes
    - InstanceGroupMember
    - InstanceGroupPolicy
    - InstanceGroupMetadata

Blueprint: db-sync-models-with-migrations

Change-Id: Ice28b9d545f60f7df381e3fcf2c86a2b8e7350d3
Merge "Sync db.models.Instance* with migrations."