Change the initialization for the Riak backend so it is consistent with how the other backends are being initialized.

Otherwise we get:

TypeError: __init__() got multiple values for keyword argument 'XXXXX'

When nydus tries to initialize the backend.
Merge branch 'riak-backend-init' of github.com:numan/nydus into numan-riak-backend-init

Conflicts:
	nydus/db/backends/riak.py