Composable service templates can now define external_update_tasks and
external_upgrade_tasks. They are meant for update/upgrade logic of
services deployed via external_deploy_tasks. The external update
playbook first executes external_update_tasks and then
external_deploy_tasks, the procedure for upgrades works
analogously. All happens within a single playbook, so variables or
fact overrides exported from the update/upgrade tasks will be
available to the deploy tasks during the update/upgrade procedure.
Partial-Bug: #1783949
Change-Id: Ib2474e8f69711cd6610a78884d5032ffd19ad249
6364f2286c