Fix IPA builds without packages specified

Since story 2006855, it is possible to specify a list of packages when
building IPA images. However, this introduced an issue in the case where
no packages are specified. See
https://github.com/stackhpc/ansible-role-os-images/issues/26.

This change fixes the issue by updating stackhpc.os-images role to
v1.7.0, which has a fix for this issue.

Change-Id: I70d20381bec3f5b92bb467e6c506e32bb13548c2
Story: 2007069
Task: 37952

releasenotes/notes/fix-ipa-build-b31fb20ddcbe3ece.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes an issue with building IPA images when no additional packages are
+    specified. See `story 2007069
+    <https://storyboard.openstack.org/#!/story/2007069>`__ for details.

requirements.yml

@@ -30,7 +30,7 @@
 - src: stackhpc.mellanox-switch
   version: v1.0.0
 - src: stackhpc.os-images
-  version: v1.6.0
+  version: v1.7.0
 - src: stackhpc.os-ironic-state
   version: v1.2.0
 - src: stackhpc.os-networks