rpc: Remove duplicated logic, fix unittests

Commit 5fce6a89 changed RpcRunner._InstDict to add the disk parameters
on all encoded instances. It didn't remove a special case in
“_InstDictOspDp”. Update and fix unittests as well.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>