fix(coreos-install): Add support for choosing channels, new bucket URLs
fix(coreos-install): Replace partprobe with blockdev.

We don't need the fancier partition scanning logic partprobe defines and
we certainly don't want the warnings it generates in the process. Simply
calling the kernel ioctl to re-read the whole disk's partition table is
sufficient since no partitions on the device should be in use.
fix(coreos-install): Switch from lsblk to blkid and search by LABEL

This fixes compatibility with old versions of util-linux because Ubuntu.
Merge pull request #99 from marineam/buckets

Buckets of fun