Don't create a partition table in the helper image

There is no need for having partitions in the helper image. It is more
convenient if we format the whole disk as an ext2 filesystem.
Shrink the helper image to the minimum size

Since the helper image is read-only there is no need to waste space.
Add btrfs-tools in the helper's packages list
Update the helper kernel to 3.14.15

Newer kernels work better with btrfs
Don't fail if /etc/fstab is missing

If /etc/fstab is missing do not mount any other partition than the
root partition but do not fail. There are distros like CoreOS that
do not contain an `/etc/fstab'.
Preserve the name when resizing GPT partitions
Detect CoreOS in get_base_distro()
Allow mounting btrfs file systems in mount_all()
Support enlarging btrfs file systems

Resizing of a btrfs file system is performed in FilesystemResizeMounted
configuration task.