Enhance master images caching for PXE driver

To speed up large deployment, Ironic Conductor requires local caching of master images downloaded from Glance. Currently, it stores image while it is used by any instance, and any newly deployed instance may also reuse this image.
This blueprint proposes the following enhancements:
* Store images for some time after they're not in use by any instance, limit cache in size and TTL (bug #1199665)
* Delete instance images (but not TFTP images) right after deployment, so that they can be deleted from cache eventually (bug #1231351)
* Consider also Glance checksum when caching. For example, add it to file name of master image in local cache, so that when image is updated in Glance, it is downloaded again and stored in cache under a different file name. This image will only be used for subsequent deployments, old deployments will still use the old image.
NOTE(dtantsur): I have doubts now that Glance does support updating active image, so I suggest withdrawing the third action item.
NOTE2: As per dicussion on IRC, the 3rd point is dropped, so there's no more point in this blueprint.