Add prepare_request function to requests.py

This function is supposed to be called to prepare a request that will be
used later by other parts of this client.

Future patches will more clearly show how this function is meant to be
used.

Implements blueprint python-marconiclient-v1

Change-Id: I08b35996119c4e12c31e3113771390b2ec58dfd3
Merge "Add prepare_request function to requests.py"