db_sync --expand may run downtime-incurring operations in upgrades to Newton

In discussing a fix for bug 1615014 with Richard, we realized that running:

  keystone-manage db_sync --expand

... automatically runs legacy (offline) migrations *specifically* when upgrading from Mitaka->Newton. Therefore, we don't technically support zero-downtime, rolling upgrades for Mitaka->Newton, despite having all the rolling upgrade commands available in Newton.

Therefore, all of the following migration paths are effectively identical in Mitaka->Newton upgrades, in that they all involve destructive migrations downtime in the first step:

Migration path A:

  keystone-manage db_sync # downtime-incurring operations normally occur here

Migration path B:

  keystone-manage db_sync # downtime-incurring operations normally occur here
  keystone-manage db_sync --expand # this becomes a no-op
  keystone-manage db_sync --migrate # this becomes a no-op
  keystone-manage db_sync --contract # this becomes a no-op

Migration path C:

  keystone-manage db_sync --expand # downtime-incurring operations only occur here in Mitaka->Newton
  keystone-manage db_sync --migrate
  keystone-manage db_sync --contract # downtime-incurring operations normally occur here

Migration path A is required for migrations up until Liberty->Mitaka, and is still supported in Ocata and beyond. Migration path B works, but there is no reason to recommend this path. Migration path C is recommended for Newton->Ocata upgrades and beyond.