The chunk_size used for downloading images was 1KiB for some time. That
is okay for relatively small images but the client side of CPU can be a
bottleneck especially for large images. Bump the default chunk_size from
1KiB to 1MiB so we can use the client side CPU more efficiently.
[1KiB chunk_size - current]
$ time openstack image save IMAGE_689MB --file /dev/null
real 0m16.633s
user 0m12.633s
sys 0m1.365s
-> ~331 Mbps
[1MiB chunk_size - patched]
$ time openstack image save IMAGE_689MB --file /dev/null
real 0m4.896s
user 0m3.361s
sys 0m0.724s
-> ~1,125 Mbps
Story: 2010759
Task: 48044
Change-Id: Ib877d292f8adbf2fa0c51065f2917b3f1e263483
a72d46a955