ConfigWriter: move _temporary_ids to reservation

In order to do this we need to pass a job id when reserving a resource.
We have one during _EnsureUUIDs because we passed it in from AddNode and
AddInstance. During config upgrade we use a fake job ID which we then
cleanup. We can delete the _CleanupTemporaryIDs code, since the cleanup
is going to be done at job finish time by mcpu.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>