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/selftest/suite_test/test_suite/mo_mt_sms.py b/selftest/suite_test/test_suite/mo_mt_sms.py
index cf44357..8b5e1ec 100644
--- a/selftest/suite_test/test_suite/mo_mt_sms.py
+++ b/selftest/suite_test/test_suite/mo_mt_sms.py
@@ -1,10 +1,10 @@
-nitb_iface = resources.nitb_iface()
+ip_address = resources.ip_address()
 nitb = resources.nitb()
 bts = resources.bts()
 ms_mo = resources.modem()
 ms_mt = resources.modem()
 
-nitb.start(nitb_iface)
+nitb.start(ip_address)
 bts.start(nitb)
 
 nitb.add_subscriber(ms_mo, resources.msisdn())