The simplest way to support this in your image is to install the cloud-utils package (contains the growpart tool for extending partitions), the cloud-initramfs-growroot package (which supports resizing root partition on the first boot), and the cloud-init package into your image. With these installed, the image performs the root partition resize on boot. For example, in the /etc/rc.local file. These packages are in the Ubuntu and Debian package repository, as well as the EPEL repository (for Fedora/RHEL/CentOS/Scientific Linux guests).
If you cannot install cloud-initramfs-tools, Robert Plestenjak has a github project called linux-rootfs-resize that contains scripts that update a ramdisk by using growpart so that the image resizes properly on boot.
If you can install the cloud-utils and cloud-init packages, we recommend that when you create your images, you create a single ext3 or ext4 partition (not managed by LVM).
