Relax requirement of diskimage-builder

Version 3.4.0 breaks bifrost on stable/victoria as the upper constraints
is set to 3.2.1, this version should be enough to work fine with centos
stream even if it doesn't officially supports it.

Change-Id: I6a278b11ed6d14661be87cfeee4bec03c42f0f9d

requirements.txt

@@ -1,4 +1,4 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-diskimage-builder>=3.4.0  # Apache-2.0
+diskimage-builder>=3.2.1  # Apache-2.0