TT#60950 Switch janus-admin docker to buster

Removing nodejs-legacy as it is not availabe in buster,
also it should be unnecessary nowadays.

It was adde in commit f3e1d8e621

> TT#22402 Switch janus-admin Dockerfile to 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.

Change-Id: I19465cfbca0cdfffe43293abffee18fac0741851

t/Dockerfile

@@ -1,20 +1,19 @@
-# DOCKER_NAME=janus-admin-stretch
-FROM docker.mgm.sipwise.com/sipwise-stretch:latest
+# DOCKER_NAME=janus-admin-buster
+FROM docker.mgm.sipwise.com/sipwise-buster: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 2017-09-25
+ENV REFRESHED_AT 2019-06-27
 
-# 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
+# TODO - the release-trunk-buster is too dynamic yet, though required build deps
+RUN echo "deb https://deb.sipwise.com/autobuild/ release-trunk-buster main" >>/etc/apt/sources.list
 
 RUN apt-get update
 RUN apt-get install --assume-yes \
         curl \
         nodejs \
-        nodejs-legacy \
         npm
 
 ADD package.json /tmp/
@@ -34,13 +33,13 @@ 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-stretch" -f ./t/Dockerfile .
-# % docker run --rm -i -t -v $(pwd):/code:ro -v /results janus-admin-stretch:latest bash
+# % docker build --tag="janus-admin-buster" -f ./t/Dockerfile .
+# % docker run --rm -i -t -v $(pwd):/code:ro -v /results janus-admin-buster:latest bash
 #
 # Use the existing docker image:
-# % docker pull docker.mgm.sipwise.com/janus-admin-stretch
+# % docker pull docker.mgm.sipwise.com/janus-admin-buster
 # 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-stretch:latest bash
+# % docker run --rm -i -t -v $(pwd):/code:ro -v /results docker.mgm.sipwise.com/janus-admin-buster:latest bash
 #
 # Inside docker:
 #   cd /code && ./t/testrunner