Make object actions pass positional arguments

This changes the Object API to also pass positional arguments
to remotable methods. Previously, we required only keyword arguments
to these methods in order to mirror our current RPC behavior.
This is not really necessary and could be confusing.

Since there are no actual users of these APIs in the tree right
now, we can make this change without needing to bump any versions.

Related to blueprint unified-object-model

Change-Id: Ifb12843d16500d383d4e9c607121d5584400f247
Merge "Make object actions pass positional arguments"