Drop default max image age to 3 days

We ship updates pretty fast, and after seven days, there are
hundreds being installed on every update test. It's gotta be a
better tradeoff to just regen the base images more often.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

createhdds.py

@@ -210,7 +210,7 @@ class VirtInstallImage(object):
     and 'rawhide' are flags for whether this is a branched or Rawhide
     release (which affects the paths).
     """
-    def __init__(self, name, release, arch, size, variant=None, imgver='', maxage=7,
+    def __init__(self, name, release, arch, size, variant=None, imgver='', maxage=3,
                  bootopts=None, branched=False, rawhide=False):
         self.name = name
         self.size = size