Add ceph_client tag to execute or skip the playbook

Add ceph_client tag to execute or skip the playbook

There are some use cases where there's a need to skip the execution
of the ceph-ansible client role even though the client section of the
inventory isn't empty.
This can happen in contexts where the services are colocated or when
a all-in-one deployment is performed.
The purpose of this change is adding a 'ceph_client' tag to avoid
altering the ceph-ansible execution flow but at the same time be able
to include or exclude a set of tasks using this tag.

Signed-off-by: Francesco Pantano <fpantano@redhat.com>