Fix image build checks missing setuptools

Use apt to install python3-pip, and use pip3 in event system has
both pip2 and pip3 installed. Use apt to install setuptools for
Ansible's consumption.

Change-Id: Id80c809c636abe41a1cbb4d465f82ed1e8e0e9d7
Signed-off-by: Alexander Hughes <Alexander.Hughes@pm.me>

python/requirements-test.txt

@@ -1,4 +1,4 @@
-pytest-mock
+pytest-mock==3.1.0
 pytest
 pytest-cov
 responses

tools/gate/playbooks/docker-image-build.yaml

@@ -47,10 +47,13 @@
           with_items:
             - docker.io
             - python-pip
+            - python-setuptools
+            - python3-setuptools
           when: ansible_os_family == 'Debian'
         - pip:
             name: docker
             version: 2.7.0
+            executable: pip
       become: True
 
     - name: Make images