Refactor image_build and image_push roles

Primary changes are:

- Generalized variable names (remove "docker")
- Add explicit "push" variable rather than checking if the "registry" variable is defined.
- Allow for passing in version as build arg

tools/ansible/roles/image_build/defaults/main.yml

@@ -1,2 +1,5 @@
 ---
-awx_image: quay.io/ansible/awx
+awx_image: ansible/awx
+awx_image_tag: "{{ awx_version }}"
+dockerfile_name: 'Dockerfile'
+