repart: make No-Auto GPT partition flag configurable too

This is useful for provisioning initially empty secondary A/B root file
systems. We don't want those to ever be considered for automatic
mounting, for example in "systemd-nspawn --image=", hence we should
create them with the No-Auto flag turned on. Once a file system image is
dropped into the partition the flag may be turned off by the updater
tool, so that it is considered from then on.

Thew new option for this is called NoAuto. I dislike negated options
like this, but this is taken from the naming in the spec, which in turn
inherited the name from the same flag for Microsoft Data Partitions. To
minimize confusion, let's stick to the name hence.