Check if the docker is running if it is required

[daniel1302]

People are confused that sometimes the nomad cannot start jobs with the following error:

Constraint missing drivers filtered 1 node

This usually means that the docker is not running.
We should check if docker is required - we can do it after the config is parsed and fails with a clear error if it is required and not running.

TODOs

• Check if docker is required and fail if it is not running and required.