The dummy client container currently wont work on non-x86_64 hosts.
This PR creates a filtered client group that contains only hosts
that are x86_64 - which can then be the group to run the
dummy container against.

This is for the specific case of a containerized_deployment where
there is a mixture of non-x86_64 hosts and x86_64 hosts. As such
the filtered group will contain all hosts when running with
containerized_deployment: false.

Currently ppc64le is not supported for Ceph server components.

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