gateway: increase image size in qemu dev test environment

[herzogf]

#20 will probably add a cache for debian packages to the gateway thus the 2gb disk size / raspbian image size is not enough.
We need to resize the image and then call raspi-config to resize the partition.

# on the host
qemu-img resize raspbian_lite_latest.img +10G
# via ansible within the gateway
raspi-config --expand-rootfs

HINT: Increasing the raspbian*.img image size with qemu-img resize raspbian_lite_latest.img +10G consumes 10 additional gb of disk space on the host system which is really annoying. Perhaps we should convert the image from img to qcow2 format which does not reserve space on the host if the disk/image space is not used.