app: make sure that saveNewUnitsInDatabase.Backward is nil
app: define MinParams in the saveNewUnitsInDatabase action
app: get the app from the database before updating it
app: check the presence of the app in DB in reserveUnitsToAdd action
app: make Provision.AddUnits atomic
api, cmd/tsr, collector: move provisioners registration to cmd/tsr
provision/docker: don't add new units to undeployed apps