Changes

debian-stretch-jenkins/Dockerfile: Install source-highlight

It is needed by some user manuals using code snippet syntax
higlhlighting, such as osmo-gsm-tester one:

"""
[source,python]
----
sleep(3) # sleep for 3 seconds
----
"""

asciidoc: WARNING: test_api.adoc: line 19: filter non-zero exit code: source-highlight -f xhtml -s python: returned 127
asciidoc: WARNING: test_api.adoc: line 19: no output from filter: source-highlight -f xhtml -s python

Change-Id: I8427179f7cfc0c9150e5327803ca89adffedb8f1