Adds get/setManagementNodeUri to ManagementContext interface

I'm seeking feedback rather than expecting this to be merged.

The setter on the interface isn't great. How about just having getManagementNodeUri on the interface and a set.. on AbstractManagementContext. Anything that wants to set (only BrooklynWebServer?) has to cast.
Use `ManagementContextInternal`, and add method there - all management contexts should implement that, so always safe to cast.