When running a scenario, `kwargs` is used as default key-word arguments.
But in some scenarios, there are more and one services being called, and
we use xxx_kwargs for this case.
However, some xxx_kwargs are not unifed for same usage[0]. Unifying
these could avoid midleading for end users. Another improvement is to
add xxx_kwargs with empty settings for scenario config files.
[0] http://paste.openstack.org/show/489505/
Change-Id: I80f49be5c1cd21812d6b2d56b0f4279f0cf4f402
99f165395a