Please build minimized images in a 1GiB image

[Balint Reczey]

[Impact]

 * Without the fix the initial size of the image is ~2.2GiB, even for minimized images. Changing it to 1GiB allows detecting an increase in packages' installed installed size in case they would exceed this smaller limit.
 * Minimized images are generated for Xenial, too, and having this limit in effect may help keeping images small.

[Test Case]

 * Generate all minimized an non-minimized images to see that all the builds still succeed.

[Regression Potential]

 * Since partitions images are expanded to pre-defined sizes at the end of image generation this change should not change the resulting image sizes where having free space is needed at run-time.
 * The change may break building images, hence the tests in the Test Cases section.
 * The change is tested extensively in Bionic

[Robert C Jennings (rcj)]

Testing must include builds of cloud and MAAS images

[Balint Reczey (rbalint)]

We discussed the change with Steve Langasek again and agreed that the initial image should not be changed and the size of the final images produced by livecd-rootfs should be monitored.
The change already made in trunk will be reverted soon.