Bump NODE_DISK to 20

I was using NODE_DISK=10 and my undercloud ran out of disk space. This
was with Fedora images, so perhaps they are larger.

Because these are thinly provisioned qcow2 disk images, it seems
worthwhile to be generous rather than cutting it tight.

Change-Id: I9829f9e969e4c3f6e76d78231461a00b1adc6917