One of our providers is struggling with Glance backend tasks after
upload, which causes increasingly lengthy delays for image readiness
the more images we're uploading. Globally reduce our upload
concurrency to the minimum possible of one per builder, for now, in
order to alleviate the delays and increase the chances images will
come ready in that provider.
This can be reverted once the situation has been resolved.
Change-Id: I29daa8f6d2d13055baf74215184ce0987fc20be0
9bbe19d4a3