stats: adding symlinks for nodes and interfaces in the stat segment

A given interface counter (e.g rx) can be accessed via
/interfaces/<interface_name>/<counter_name>.
Same goes with nodes: /nodes/<node_name>/<counter_name>
As interfaces may contain '/' in their names,
these are replaced by '_' in symlinks
Also added 2 tests for the stat segment

Type: feature

Signed-off-by: Arthur de Kerhor <arthurdekerhor@gmail.com>
Change-Id: I27da252e7b3dc177815616ca46b5c200a456bf0f
Signed-off-by: Ole Troan <ot@cisco.com>