Add dedicated NodeRole.ControlService to distinguish it from ordinary hosts

[amdfxlucas]

currently SCION ControlServices (CS) are treated like end hosts by SEED (they are registered under NodeRole.Host).

However this is sub optimal for at least the following reasons:

• a SCION simulation always requires at least a CS and border router (BRD) node
  but can make do without a single host (i.e. transit AS)
• a designated 'type' would ease retrieval/lookup of CS nodes from the registry and facilitate the special treatment it deserves (i.e. branching on node type to select SCION distributables that need to be installed )
• would enable 'assertions' on node type

Accordingly this PR proposes to add a NodeType for CS'es

[lschulz]

Introducing a new node role might have a big impact on the entire codebase. In contrast to introducing BorderRouter in contrast to Router, the change would be specific to SCION. The CS has no equivalent in BGP.

The cases you quote can all be solved in different ways as far as I can see. I'm not convinced treating the CS extra special is the right way to go.

We should also not forget that the CS is actually multiple services, the certificate server, beacon server, path server, etc. are all distinct. The fact that they are implemented in the same process is just an implementation detail. By the same logic, we could consider introducing node roles for each of the services, but since each Node has only exactly one role that wouldn't work without further changes.

In summary, I would prefer a different solution like adding some kind of label or attribute to the host node.

@martenwallewein, @tjohn327, @LencigGaer What is your opinion?

[amdfxlucas]

That's not the deciding reasoning behind NodeRoles anyway ( there's a RouteServer role which is very specific to BGP )
Ultimately they appear to exist for convenience and verifiability . I see the point that a node can provide multiple services, but i arose only resistance when i proposed to make NodeRole into an EnumFlag, so roles could be seamlessly combined (AND/OR -ed and tested so the branching simplifies ). They're not willing to reify this fact in their code.