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/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg b/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg
index 6b48e97..e835e06 100644
--- a/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg
+++ b/doc/examples/osmo-bsc_nat/osmo-bsc_nat.cfg
@@ -62,5 +62,5 @@
  timeout ping 20
  timeout pong 5
  ip-dscp 0
- bscs-config-file bscs.config
+ bscs-config-file bscs.cfg
  access-list bla imsi-allow ^11$