Refactor ShowInterfacesCommand in ChassisCapability.

It now uses ModelHelper to retrieve interfaces from the model.
It now uses getResourceFromFriendlyName() in GenerikKarafCommand to get the resource the command applies to.
Printing code is now inside printing methods, not inside doExecute().

Functionality has not changed and the command prints exactly the same it was printing before this patch.
Place logic to retrieve GRE interfaces from the model in a router ModelHelper.

ShowInterfacesCommand uses ModelHelper to retrieve GRE "interfaces".

TODO: Investigate why there is only one greService taken into account.
Merge pull request #796 from isartcanyameres/refactor/router-getinterfaces-common-logic

Refactor/router getinterfaces common logic