fix make distcheck with python tests

- bscs.config needed by the vty tests was not picked up as a dist file, because
  its suffix is not 'cfg'. Rename to *.cfg. Apply this rename in
  vty_test_runner.py and osmo-bsc_nat.cfg.
- Remove restart counters after external tests, otherwise distcheck complains
  about uncleaned files.
- Add contrib/ipa.py to EXTRA_DIST, hence add a Makefile.am to contrib/.
  Otherwise the python tests cannot find that dependency.

Change-Id: I42b55cb1125099afc3a8e3f87c0e398426b2e2a9
diff --git a/tests/vty_test_runner.py b/tests/vty_test_runner.py
index 92775d5..68a697d 100644
--- a/tests/vty_test_runner.py
+++ b/tests/vty_test_runner.py
@@ -1220,7 +1220,7 @@
 def nat_bsc_reload(x):
     x.vty.command("configure terminal")
     x.vty.command("nat")
-    x.vty.command("bscs-config-file bscs.config")
+    x.vty.command("bscs-config-file bscs.cfg")
     x.vty.command("end")
 
 def nat_msc_ip(x, ip, port):