Add inventory builder python script

Includes tox support for running unit tests.
Small note added to getting-started guide for using
inventory_builder.py

Also adds manual-only unit test.

.gitlab-ci.yml

@@ -363,3 +363,12 @@ syntax-check:
   script:
     - ansible-playbook -i inventory/local-tests.cfg -u root -e ansible_ssh_user=root  -b --become-user=root cluster.yml -vvv  --syntax-check
   except: ['triggers']
+
+tox-inventory-builder:
+  stage: unit-tests
+  <<: *job
+  script:
+    - pip install tox
+    - cd contrib/inventory_builder && tox
+  when: manual
+  except: ['triggers']