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/Makefile.am b/Makefile.am
index 31a54ea..4a1f65d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -13,6 +13,7 @@
 	include \
 	src \
 	tests \
+	contrib \
 	$(NULL)
 
 pkgconfigdir = $(libdir)/pkgconfig