wget http://gaby.rohost.com/curtin-img-builder.tar.gz

tar -xzvf curtin-img-builder.tar.gz
cd curtin-img-builder/bin

ls -l build    # our script

# to be used on ubuntu with virtualization capabilities
sudo tasksel # select virtual machine host

# for any other prerequisites the script may require
sudo ./build --install-deps

The default mirrors make 200 kbps seem dreamy; change them from this file if you're not getting any younger:

curtin-image-builder/contrib/centos/centos7/centos7-amd64.ks   # lines 5 to 8

You can find CentOS mirrors here.

# basic usage example:
sudo ./build --vcpus 2 -r 4096 -o /root/images --centos-edition 7 centos amd64

# run as the MaaS user:
maas root boot-resources create name=centos/7 architecture=amd64/generic filetype=ddtgz content@=/path/to/file