Split get_transport into 2 different functions

This patch splits get_transport into:

    - get_transport: Basic transport loading that does not depend
    on a request.
    - get_transport_for: Transport loading based on a request object.

Partially-Implements blueprint python-marconiclient-v1

Change-Id: Id1adfb3fc3d2914bf81e4b204ca3eae82a42b94d