remove(build_container): Kill catalyst-based container script

Never actually used this, new plan is slightly different.
fix(disk_util): Don't try to format blank partitions.
fix(disk_util): Set btrfs default subvolume

This avoids needing to always pass the subvol=root mount option.
fix(build_library): Skip modifying non-existent partitions.

To make it possible to plop a CoreOS install into a simple
single-filesystem image for use as a container some things like
configuring bootloaders need to be skipped.
feat(build_image): Add 'container' image type.

This image type is the same as the developer image except that it is a
single root filesystem and is bootable via systemd-nspawn. This may
become obsolete eventually when it becomes possible to boot the normal
disk images under nspawn but it is useful for testing until then.

The partition type is defined by the Discoverable Partitions Spec.
http://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/
Merge pull request #253 from marineam/dev-container

nspawn compatible dev container