TT#22402 Switch janus-admin Dockerfile to Debian stretch

We have to install 'npm' explicitly here as it is no longer provided
by package 'nodejs' in Debian stretch.

We have to install 'nodejs-legacy' otherwise /usr/bin/node
is not available and test 'janus-admin-unit-test-docker' fails:

> + docker run --rm -i --entrypoint /bin/bash -v /var/lib/jenkins/workspace/janus-admin-unit-test-docker/source/:/code:ro \
>   -v /var/lib/jenkins/workspace/janus-admin-unit-test-docker/results:/results:rw \
> docker.mgm.sipwise.com/janus-admin-stretch:I3370beedf30495497d01ec42eb292f90bce2b0a2 -c /code/t/testrunner
> ### Copying and moving files
> ################################################################################
> Finished main setup, now running tests ...
>
> > janus-admin@1.0.4 test /tmp/code
> > mocha -R spec --full-trace test/*-spec.js
>
> /usr/bin/env: 'node': No such file or directory
> npm ERR! Test failed.  See above for more details.
> npm WARN This failure might be due to the use of legacy binary "node"
> npm WARN For further explanations, please read
> /usr/share/doc/nodejs/README.Debian
>
> npm ERR! not ok code 0

You can find more information about node/nodejs name convention here:
https://lists.debian.org/debian-devel-announce/2012/07/msg00002.html

So far, we need to install 'nodejs-legacy' to provide /usr/bin/node.

Change-Id: I3370beedf30495497d01ec42eb292f90bce2b0a2

t/Dockerfile

@@ -1,19 +1,21 @@
-# DOCKER_NAME=janus-admin-jessie
-FROM docker.mgm.sipwise.com/sipwise-jessie:latest
+# DOCKER_NAME=janus-admin-stretch
+FROM docker.mgm.sipwise.com/sipwise-stretch:latest
 
 # Important! Update this no-op ENV variable when this Dockerfile
 # 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 2017-05-09
+ENV REFRESHED_AT 2017-09-25
 
-# TODO - the release-trunk-jessie is too dynamic yet, though required build deps
-RUN echo "deb https://deb.sipwise.com/autobuild/ release-trunk-jessie main" >>/etc/apt/sources.list
+# TODO - the release-trunk-stretch is too dynamic yet, though required build deps
+RUN echo "deb https://deb.sipwise.com/autobuild/ release-trunk-stretch main" >>/etc/apt/sources.list
 
 RUN apt-get update
 RUN apt-get install --assume-yes \
         curl \
-        nodejs
+        nodejs \
+        nodejs-legacy \
+        npm
 
 ADD package.json /tmp/
 ADD npm-shrinkwrap.json /tmp/
@@ -32,13 +34,13 @@ WORKDIR /code
 # ----------------------
 # When you want to build the base image from scratch (jump to the next section if you don't want to build yourself!):
 # NOTE: run the following commands from root folder of git repository:
-# % docker build --tag="janus-admin-jessie" -f ./t/Dockerfile .
-# % docker run --rm -i -t -v $(pwd):/code:ro -v /results janus-admin-jessie:latest bash
+# % docker build --tag="janus-admin-stretch" -f ./t/Dockerfile .
+# % docker run --rm -i -t -v $(pwd):/code:ro -v /results janus-admin-stretch:latest bash
 #
 # Use the existing docker image:
-# % docker pull docker.mgm.sipwise.com/janus-admin-jessie
+# % docker pull docker.mgm.sipwise.com/janus-admin-stretch
 # NOTE: run the following command from root folder of git repository:
-# % docker run --rm -i -t -v $(pwd):/code:ro -v /results docker.mgm.sipwise.com/janus-admin-jessie:latest bash
+# % docker run --rm -i -t -v $(pwd):/code:ro -v /results docker.mgm.sipwise.com/janus-admin-stretch:latest bash
 #
 # Inside docker:
 #   cd /code && ./t/testrunner