Add ability to use a different client container

Add ability to use a different client container

Currently a throw-away container is built to run ceph client
commands to setup users, pools & auth keys. This utilises
the same base ceph container which has all the ceph services
inside it.

This PR allows the use of a separate container if the deployer
wishes - but defaults to use the same full ceph container.

This can be used for different architectures or distributions,
which may support the the Ceph client, but not Ceph server,
and allows the deployer to build and specify a separate client
container if need be.

Signed-off-by: Andy McCrae <andy.mccrae@gmail.com>