Make CommandManager::AddCommand() usable for both cloud and local cases

Add Command::Origin parameter to AddCommand as well as let it return
the cloud command ID for the command instance.

Change-Id: I694c72aba80bc0f0f240453bfab0d11e773d70aa
Reviewed-on: https://weave-review.googlesource.com/1786
Reviewed-by: Vitaly Buka <vitalybuka@google.com>