add first draft of pseudo code for data plane upgrade

[rgildein]

The idea behind this approach is to handle all the machines and applications in them through Nova-compute applications. That way, not much code will change, what is benefit.

I prepared a diagram for a more detailed description.

Known issues:

• Nova-compute application needs to be responsible for upgrading apps deployed under the same machine.
  • Steps like config changes must be done as pre-checks in nova-compute for all apps.

[rgildein]

After review, we decide to go another direction.