Currently all almost all of the deployment process is contained in a
single deploy step called deploy, on the deploy interface. This
restricts the customisation that can be applied via deploy steps, since
steps may only be added before or after this step.
This change allows deploy interfaces inheriting agent_base.AgentDeployMixin
to be used with in-band deploy steps. It is implemented by decomposing
the reboot_and_finish_deploy call into three deploy steps:
* tear_down_agent (priority 40)
* switch_to_tenant_network (priority 30)
* boot_instance (priority 20)
All steps with priorities between 99 and 41 can run in-band.
Backwards compatibility with drivers that do not support decomposed
steps is maintained via a 'has_decomposed_deploy_steps' method on the
agent heartbeat mixin.
The old reboot_and_finish_deploy call is also left for compatibility
but does nothing since the new steps will be automatically run.
Change-Id: Ie4fdd195efa941165e22bd4ce9484988a1760b2d
Story: #2006963
Task: #40151
deec7f4a92