Ensure pip is installed on zuul executor

Also add mock to test requirements

Change-Id: I016238e927f3c012fcc2b1510a1671003b839da8
Signed-off-by: Andrii Ostapenko <andrii.ostapenko@att.com>

test-requirements.txt

@@ -14,4 +14,5 @@ sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
 testfixtures>=5.1.1 # MIT
 stestr>=1.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT
+mock>=2.0
 mysql-connector-python==8.0.17

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

@@ -1,4 +1,6 @@
 - hosts: all
+  roles:
+    - ensure-pip
   tasks:
     - include_vars: vars.yaml
 
@@ -33,7 +35,6 @@
             allow_unauthenticated: True
           with_items:
             - docker-ce
-            - python-pip
         - pip:
             name: docker
             version: 2.7.0