Allow software configuration at deploy time

For heat sw-config, it's preferable to deploy SoftwareConfig elements at instance deploy time if possible, since it saves significant time and complexity as os-collect-config has to rerun in its entirety. This is different from the murano-agent model which relies on the instance being available to receive instructions before sending those instructions.

This is a two-stage proposal: the first is to allow HeatSWConfigInstances to inject software components at deploy time. This would not change the public API of the Instance model, but would allow additional template components to be injected atdeploy time (versus afterwards with stack-update).

The second stage (if necessary) is to abstract this further, such that a class provides an interface whereby softwareconfig changes are published by an application, but it doesn't necessarily know how; it would be up to the softwareconfig provider (determined by a subclass of Instance) how exactly to publish (as heat sw, or murano-agent). This work would likely be more involved and need more design, and should likely be moved to a separate BP if necessary.