Pass keywords to the request initialization

The `prepare_request` function accepts keywords corresponding to what
the Request class can take as arguments. This patch passes those kwargs
to the newly created `Request` instance, which is needed for the correct
work of the auth backend.

Partially-Implements blueprint python-marconiclient-v1

Change-Id: Ib333a7ad4d4628e2e5a66ddc91fafc378679f69c
Merge "Pass keywords to the request initialization"