Remove wrong equal sign

According to the official doc[1], the syntax was wrong, leading to
issues. It wasn't detected until now, because the option wasn't
actually used.

[1] https://docs.docker.com/engine/reference/builder/#arg

Change-Id: Icdbc8d687e1a19ec4015c57acc05d35ddcfd2a77
(cherry picked from commit e9a1797366)

tripleo_ansible/roles/tripleo_container_image_build/templates/Containerfile.j2

@@ -14,7 +14,7 @@
 # under the License.
 
 {% for key, value in tcib_args.items() %}
-ARG = {{ key }}={{ value | to_json }}
+ARG {{ key }}={{ value | to_json }}
 {% endfor %}
 FROM {{ tcib_from }}
 {% for key, value in tcib_labels.items() %}