Allow overriding the Ceph client setup via attribute

This change introduces the attribute

    node['openstack']['ceph']['setup_client']

that lets the user override all actions of the ceph_client recipe.

It addresses the usecase where you use the (github) ceph/ceph-cookbooks
for setting up your nodes and don't want openstack-common to manage and
hence override your config.

The way this has been implemented is modelled after the
openstack-network plugin recipes (e.g. openstack-network::linuxbridge)
which just return when the proper attribute is not set.  This way, we
can still have all those recipes that need a working Ceph client
environment just include_recipe openstack-common::ceph_client, without
placing conditionals there.

Change-Id: I491feecae1ed35871b11add334df0df6fd413d1e