So far the tasks for external update/upgrade were not using the step
mechanism as other tasks, we had a single step. As external
deploy/update/upgrade tasks are being used for more things nowadays,
it's likely that we'll need to go towards a similar model like we have
for deploy/update/upgrade tasks -- proper usage of steps.
For now we have just 2:
* Step 0 for setting global facts, and performing validations.
* Step 1 for actual update/upgrade tasks. (There's an upcoming change
to run online data migrations in step 1).
Change-Id: I1933bd0eedab71caab56c0e5d93ba7927fb7c20f
Partial-Bug: #1793332
bcd6cde608