Switch back to python 3.7

Ansible 2.7 is known to be broken with python 3.8 so switch back to
python 3.7 for now until we dropped support for Ansible 2.7.

Change-Id: Ibfe1b226630c8db202a294590c6435666afcb62d

.zuul.yaml

@@ -130,8 +130,8 @@
     allowed-projects: zuul/zuul
     timeout: 2700  # 45 minutes
     requires:
-      - python-builder-3.8-container-image
-      - python-base-3.8-container-image
+      - python-builder-container-image
+      - python-base-container-image
     provides: zuul-container-image
     vars: &zuul_image_vars
       docker_images:
@@ -160,8 +160,8 @@
     description: Build Docker images and upload to Docker Hub.
     allowed-projects: zuul/zuul
     requires:
-      - python-builder-3.8-container-image
-      - python-base-3.8-container-image
+      - python-builder-container-image
+      - python-base-container-image
     provides: zuul-container-image
     secrets:
       name: docker_credentials

Dockerfile

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM docker.io/opendevorg/python-builder:3.8 as builder
+FROM docker.io/opendevorg/python-builder as builder
 
 # Optional location of Zuul API endpoint.
 ARG REACT_APP_ZUUL_API
@@ -39,7 +39,7 @@ RUN mkdir /tmp/openshift-install \
   && echo $OPENSHIFT_SHA /tmp/openshift-install/openshift-client.tgz | sha256sum --check \
   && tar xvfz openshift-client.tgz --strip-components=1 -C /tmp/openshift-install
 
-FROM docker.io/opendevorg/python-base:3.8 as zuul
+FROM docker.io/opendevorg/python-base as zuul
 
 COPY --from=builder /output/ /output
 RUN /output/install-from-bindep \