Use EXT2 for the rootfs images instead of CPIO

[JJL772]

It seems like CPIO doesn't support xattrs and thus loses capabilities set on files via setcap. ping, for example, currently yields Operation not permitted unless you set sudo sysctl net.ipv4.ping_group_range="0 2147483647", even though it was properly setcap'ed before being packed into the CPIO rootfs.

Before packing the CPIO rootfs, getcap centos7-builder/diskless-root/bin/ping (from inside the build container) shows: centos7-builder/diskless-root/bin/ping = cap_net_admin,cap_net_raw+p

After booting the image in Qemu, I see: Failed to get capabilities of file /bin/ping' (Operation not supported)`

It's worth noting that our linuxRT images do not have this issue because they already use EXT2 rootfs images.

Linux docs about initrd images: https://docs.kernel.org/admin-guide/initrd.html