By default we should not expose the NOMAD_LICENSE environment variable
to tasks.

Also refactor where the DefaultEnvDenyList lives so we don't have to
maintain 2 copies of it. Since client/config is the most obvious
location, keep a reference there to its unfortunate home buried deep
in command/agent/host. Since the agent uses this list as well for the
/agent/host endpoint the list must be accessible from both command/agent
and client.