Revert to uwsgi-plugin-python for barbican source images

Stein only

I508bb082a319955b3e008235e158eaa07a1e8a74 was backported to stein to fix
an issue with barbican images which contained the python 2 version of
uwsgi-plugin-python on Ubuntu/Debian, but should have used
uwsgi-plugin-python3. However, in Stein only Ubuntu/Debian binary images
used python 3 - not source images.

This change reverts to uwsgi-plugin-python for Ubuntu/Debian source.

Change-Id: I62c1a42c29ce7273028f99edbb77f4362a2452ce
Related-Bug: #1849128

Author: markgoddard

docker/barbican/barbican-base/Dockerfile.j2

@@ -25,7 +25,7 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
         {% endif %}
 
     {% elif base_package_type == 'deb' %}
-        {% set barbican_base_packages = ['uwsgi-plugin-python3'] %}
+        {% set barbican_base_packages = ['uwsgi-plugin-python'] %}
     {% endif %}
 
 {{ macros.install_packages(barbican_base_packages | customizable("packages")) }}