Once upon a time, a small systemd service was prepared to hardcode a list
of containers to launch once Docker is up. See D20 and T467.
On docker-002, we've now with Sentry ± 200 order conditions to solve.
Also on docker-002, we've still services defined with links between containers.
In such case, Docker will simply refuse to start container B as long as
the container A isn't there to satisfy "B has a link A". A correct list
is so vital in that case.
Containers are now defined in pillar, under docker_containers key.
There are a lot of configuration keys there like mysql_link that
can be interpreted as, for example, "devcentral depends of acquisitariat".
Dependencies between services are so documented by this change
under docker_containers_dependencies pillar entry, shared between all
Docker engines.
docker-paas-list-containers can then be used to query both pillar entries,
build a graph of dependencies and solve a topological order for the graph.
Fixes T1428.