Add a Client 'instantiator'

This function mocks a class constructor but returns a client instance
for the specified version. This reduces the number of imports needed to
use the API and makes it easier to migrate from one version to the
other.

Change-Id: I20bb9adef4fa4f6bb87c53020454c791d765c389
Partially-Implements: python-marconiclient-v1
Merge "Add a Client 'instantiator'"