Fix mis-scoped Client class breaking CLI

Fixes bug 1180439

The change in importation in this commit:

  https://github.com/openstack/python-ceilometerclient/commit/453f4d2a

caused the ceilometer CLI to fail in all cases with:

  'module' object has no attribute 'Client'

due to the mis-scoping of ceilometerclient.client.Client
as ceilometerclient.Client.

Change-Id: I5ec30bf74233f758519810629763ccb70bb7af87