Force DIB_PYTHON_VERSION to be 2

According to the bug, this is required to build images on RHEL 7.3,
and since we don't support python 3 yet we might as well just set
it for all images on all distros.

Change-Id: I6de108d166924e6b4d63bc3decf1ffbe94b34654
Closes-Bug: 1651285

image-yaml/overcloud-images.yaml

@@ -37,6 +37,8 @@ disk_images:
       - python-heat-agent-json-file
     options:
       - "--min-tmpfs 5"
+    environment:
+      DIB_PYTHON_VERSION: '2'
   -
     imagename: ironic-python-agent
     arch: amd64
@@ -56,4 +58,5 @@ disk_images:
       - python-hardware-detect
     options:
       - "--min-tmpfs=5"
-
+    environment:
+      DIB_PYTHON_VERSION: '2'