app: preparing for AddUnits rewrite

I'll use atomic actions, since we're going to make
dockerProvisioner.AddUnits fail when there aren't any running container.
app: implement Backward in the reserveUnitsToAdd action

Related to the last commit, and the atomic execution of App.AddUnits.
app: define MinParams in reserveUnitsToAdd
app: starting to write another AddUnits action

This action will create all units in the provisioner, using the result
from the previous action.

What's next:

	1. Backward and MinParams for provisionAddUnits
	2. Save units in the database
	3. Refactor App.AddUnits