bootdisk: Do not fail if there's no id for any device

If no device has an id, then /dev/disk/by-id doesn't exist (here, at
least), and an exception is raised when listing the content of that
directory. This results in node[:crowbar_wall][:boot_device] never being
set and therefore a failure during install if the boot disk is not sda.

This fixes bootdisk on my test setup (qemu/kvm doesn't set id for disks
by default).
bootdisk: Save boot device by path, instead of by device if no id

The path is still more stable than the device name.
Merge pull request #128 from vuntz/bootdisk-no-id

bootdisk: Do not fail if there's no id for any device