TT#37401 Ensure to always have netcat-openbsd available

We have a hard dependency on netcat-openbsd, see:

| commit 26ba0340b6
| Author: Alexander Lutay <alutay@sipwise.com>
| Date:   Mon Jun 4 13:22:27 2018 +0200
|
|     TT#37401 Fix 50ecc1544: depends on netcat-openbsd since we use 'nc -U' (not available in 'netcat-traditional')

But our docker image still uses netcat-traditional.
Make sure to have netcat-openbsd available, while at
it also adjust Build-Depends accordingly.

Change-Id: I1d3cfd9b4b56047fa51c3ef1d77060122f4d2568

debian/control

@@ -26,7 +26,7 @@ Build-Depends:
  libsys-hostname-long-perl,
  libtemplate-perl,
  libyaml-libyaml-perl,
- netcat-openbsd | netcat,
+ netcat-openbsd,
  pkwalify,
  python3-pytest,
  xsltproc,

t/Dockerfile

@@ -5,12 +5,12 @@ FROM docker.mgm.sipwise.com/sipwise-stretch:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT 2017-09-22
+ENV REFRESHED_AT 2018-11-01
 
 RUN apt-get update
 
 # sourcecode test dependencies
-RUN apt-get install --assume-yes netcat libhash-merge-perl libtemplate-perl \
+RUN apt-get install --assume-yes netcat-openbsd libhash-merge-perl libtemplate-perl \
   libyaml-libyaml-perl libyaml-tiny-perl libyaml-perl \
   libdata-validate-ip-perl libio-interface-perl libregexp-ipv6-perl
 # misc stuff for execution/debugging/devel