Move vars/common.yaml to main.yaml for buildimages and containers

These are used as roles with include_role so having common.yaml
brings no benefit since vars_from defaults to main.yaml.

So we can remove the explicit include_vars for common. First
noticed it working in [1] and then split this change as the parent
into its own review

[1] https://review.opendev.org/692985

Change-Id: Ia5c83f1bafc23d149bcc67d4af697f3af7aa976c

roles/build-containers/tasks/main.yaml

@@ -1,8 +1,4 @@
 ---
-- name: Include common vars
-  include_vars:
-    dir: "vars"
-
 - name: Set branch for building containers check jobs
   set_fact:
     ci_branch: "{{ zuul.branch | replace('stable/','') }}"

roles/build-containers/tasks/post.yaml

@@ -1,8 +1,4 @@
 ---
-- name: Include common vars
-  include_vars:
-    dir: "vars"
-
 - name: Grab job artifacts
   become: true
   args:

roles/build-containers/tasks/pre.yaml

@@ -1,8 +1,4 @@
 ---
-- name: Include common vars
-  include_vars:
-    dir: "vars"
-
 - name: Ensure legacy workspace directory
   file:
     path: '{{ workspace }}'

roles/oooci-build-images/tasks/main.yaml

@@ -1,8 +1,4 @@
 ---
-- name: Include common buildimage vars
-  include_vars:
-    file: "common.yaml"
-
 # The python_v fact is used with vars/common.yaml to set the right
 # overcloud-images file in the build-images.sh.j2 template
 - name: Get python_v fact for py2 or py3 common vars

roles/oooci-build-images/tasks/pre.yaml

@@ -1,8 +1,4 @@
 ---
-- name: Include common buildimage vars
-  include_vars:
-    file: "common.yaml"
-
 - when: rhel_image_source is defined
   block:
     - name: Set dib_local_image and other facts used by tripleo-ci build-image role build template