This adds a note to the upgrade guide for Ocata -> Pike,
to use the 'ironic-dbsync online_data_migrations'
command from the latest (9.1.1 or newer) pike releases,
due to a race-condition bug.
Change-Id: I4e9bf902c7f7083278831060cde4cd9d3f152e90
Related-Bug: 1715190