docker: use a better method to pull images

docker: use a better method to pull images

We changed the way we declare image.
Prior to this patch we must have a "user/image:tag"
format, which is incompatible with non docker-hub registry where you
usually don't have a "user". On the docker hub a "user" is also
identified as a namespace, so for Ceph the user was "ceph".

Variables have been simplified with only:

* ceph_docker_image
* ceph_docker_image_tag

1. For docker hub images: ceph_docker_name: "ceph/daemon" will give
you the 'daemon' image of the 'ceph' user.

2. For non docker hub images: ceph_docker_name: "daemon" will simply
give you the "daemon" image.

Infrastructure playbooks have been modified as well.
The file group_vars/all.docker.yml.sample has been removed as well.
It is hard to maintain since we have to generate it manually. If
you want to configure specific variables for a specific daemon simply
edit group_vars/$DAEMON.yml

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1420207
Signed-off-by: Sébastien Han <seb@redhat.com>