Systemctl service management unification

[pedronis]

@ijohnson

start-snap-services and stop-snap-services start tracking internally to the task what services are to be started/stopped, etc. (implementation detail: if this isn’t specified in the task, then do the right thing and assume “all” with the current behavior so that we can leave as much as possible in snapstate_test.go well enough alone )

given an optional “services” parameter attached to those tasks (missing/empty meaning all) and go from there seems fine to me.