[BZ 955816] Ask server for last known metric val if not known otherwise.

Not sure if it is the best idea, but it surely is the simplest to implement to solve the long standing issue described in the above BZ that can be summarized as:

If the value of a trait is not known on the agent, bundle deployment to a destination based on the value of that trait will fail until the value of the trait is read on the agent.

The fix merely tries to ask the server for the last known value of the trait if all agent-side attempts to obtain it fail.

Of course, the deployment will still fail if the value is not cached in the agent, the resource is down AND the server doesn't know any prior value of it. I guess we can safely fail in that case because we cannot magically pull the value out from the thin air.