Change to bunzip2 decompression of images

Images stored in http://storage.core-os.net/coreos are bzip2-compressed.
Fix typo in config.py

Rackspace spelled incorrectly
Mount ROOT-A partition

According to the disk layout, ROOT-A is partition 3. In builds, it is
this that is populated with the root filesystem.
Copy resolv.conf to /run

/etc/resolv.conf is a dangling symlink in the rescue environment.
It points to /run/resolv.conf, so put the good copy in there.