automation: only iterate over our AMIs

We can't delete AMIs that we don't own. Iterating over other
AMIs won't work and slows down execution.

Differential Revision: https://phab.mercurial-scm.org/D6283

contrib/automation/hgautomation/aws.py

@@ -490,7 +490,7 b" def remove_resources(c, prefix='hg-'):"
 
     terminate_ec2_instances(ec2resource, prefix=prefix)
 
-    for image in ec2resource.images.all():
+    for image in ec2resource.images.filter(Owners=['self']):
         if image.name.startswith(prefix):
             remove_ami(ec2resource, image)