rename resource nitb_iface to ip_address

I would like to use the IP addresses also for OsmoBSC processes, so it is more
than clear now that 'nitb_iface' was the wrong naming choice.

The only distinction we may need in the future is public versus loopback
interface. To add that, we may add a trait to the 'ip_address' resource
like:

  ip_address:
  - addr: 10.42.42.1
    type: public
  - addr: 127.0.0.1
    type: loopback

This way we can substitute public vs loopback addresses flexibly (e.g. using
scenarios).

Change-Id: I3ad583ae7a33f7a7bb56fe78a125f73c56a0e860
diff --git a/example/resources.conf b/example/resources.conf
index 4cc06d0..8b61248 100644
--- a/example/resources.conf
+++ b/example/resources.conf
@@ -1,6 +1,6 @@
 # all hardware and interfaces available to this osmo-gsm-tester
 
-nitb_iface:
+ip_address:
 - addr: 10.42.42.1
 - addr: 10.42.42.2
 - addr: 10.42.42.3