ci-workers: Fail if Python is not version 2.7

Ansible works with Python 2.6 and 2.7, but Buildbot requires 2.7. Both have
at least preliminary Python 3 support, but until our RPC tests are migrated to
Python 3, we need to use Python 2 for testing, and so this requirement stands.

contrib/ci-workers/unix.yml

@@ -19,7 +19,7 @@
       prompt: "Buildbot worker password (provided by ZECC)"
 
   pre_tasks:
-    - name: Install Python 2 for Ansible and Buildbot
+    - name: Install Python 2.7 for Ansible and Buildbot
       raw: test -e /usr/bin/python || test -e /usr/bin/python2 || test -e /usr/bin/python2.7 || test -e /usr/local/bin/python2.7 || (test -e /usr/bin/apt && apt -qqy update && apt install -qqy python) || (test -e /usr/bin/dnf && dnf install -qqy python2) || (test -e /usr/sbin/pkg && pkg install -qqy python2)
       register: output
       changed_when:
@@ -34,7 +34,7 @@
 
     - name: Fail if configured Python is unavailable
       fail:
-        msg: Python is not accessible at {{ ansible_python_interpreter }} on this host! Please set the inventory variable 'ansible_python_interpreter' to the location of the Python 2 binary.
+        msg: "Python is not accessible at {{ ansible_python_interpreter }} on this host! Please set the inventory variable 'ansible_python_interpreter' to the location of the Python 2.7 binary."
       when: ansible_python_interpreter is defined and python_check.rc == 1
 
     - name: Check if Python is in the default location
@@ -45,12 +45,17 @@
 
     - name: Fail if default Python is unavailable
       fail:
-        msg: Python is not accessible at /usr/bin/python on this host! Please set the inventory variable 'ansible_python_interpreter' to the location of the Python 2 binary.
+        msg: Python is not accessible at /usr/bin/python on this host! Please set the inventory variable 'ansible_python_interpreter' to the location of the Python 2.7 binary.
       when: ansible_python_interpreter is undefined and python_check.rc == 1
 
     - name: Gathering Facts
       setup:
 
+    - name: Fail if Python is the wrong version
+      fail:
+        msg: "The Python binary at {{ ansible_python.executable }} is version {{ ansible_python_version }}! Please set the inventory variable 'ansible_python_interpreter' to the location of the Python 2.7 binary."
+      when: ansible_python.version.major != 2 or ansible_python.version.minor != 7
+
   tasks:
     - name: Get dependencies for distribution
       include_vars: "{{ item }}"