Add parallel_image_downloads option

Updates the Ironic image_cache module so that we
use a new parallel_image_downloads (False by default)
option to control whether images are downloaded
and converted to raw format in parallel.

This option helps resolve IO thrashing issues which
can occur on small seed VMs (or small baremetal nodes)
where parallel image downloads and qemu-img conversions
can cause cryptic deployment failures in ironic-conductor.

Change-Id: Id1a63d22cdfd26e9360fdfd747162f549f95da33
Closes-bug: #1322676