This modifies the backfill_version_column method that was used in Pike,
to backfill the version column for the Conductor object, since we missed
setting the Conductor's version column in Pike. This method is invoked
when the operator runs ``ironic-dbsync online_data_migrations``.
Change-Id: I42fec0ac9c8b684219f14822648e627fce529d36
Partial-Bug: 1526283