ssa.c: Add ssa_node_type_str() method for all layers

Instead of having 2 separate methods:
 - core_node_type_str()
 - distrib_node_type_str()

We will have a single ssa_node_type_str() routine.

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>