Re-add accidentally removed plugins/README.md text
Different volume_size defaults for EC2 and Google

In keeping with Google Compute Engine's other images, default to 10 GB
for our Debian images. Image size is handled differently for Amazon EC2,
so leave the pre-existing 1 GB default alone there.

Also update the comment in the affected build-debian-cloud section to
match reality (the --filesystem option already exists).