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_push/defaults/main.yml

@@ -0,0 +1,4 @@
+---
+registry: quay.io
+awx_image: ansible/awx
+awx_image_tag: "{{ awx_version }}"