Prior to this change, images were being scrubbed serially while
the image locations, if multiple, were being scrubbed parallely.
For the general case, this may not achieve much performance gain as
the number of images is likely to be more than the number of image
locations per image. Hence, this change attempts to parallelize
image scrubbing while leaving image locations to be scrubbed
serially.
Also, though eventlet greenpool was being used, greening the world
wasn't done. As is, it's unlikely to achieve the intended gains. So,
this change also monkey patches essential python modules.
Finally, this change also makes the pool size configurable. This
offers the flexibility to choose between serial or parallel
scrubbing. Also, parallel scrubbing can be regulated to a desired
level by altering the pool size.
DocImpact
Implements: blueprint scrub-images-in-parallel
Change-Id: I5f18a70cd427e2c1e19a6bddeff317a46396eecc
843d43355a