Fixes files ordering when choosing newest image

For example if there are following kernels in undercloud/overcloud
image:
/boot/vmlinuz-3.9.5-301.fc19.i686.PAE
/boot/vmlinuz-3.10.10-200.fc19.i686.PAE
then disk-image-get-kernel picks vmlinuz-3.9.5. It should use the
newest one.

Change-Id: I7bbf06705e85370d66c7dd8a5d4f8d6c93b21c0c
Fixes: bug #1224365