compose: Make 'bootstrap_packages' actually optional now

It has in practice been optional since:
827e711eb74135f075f02f5c44685e4e35c1a8b1

Now let's let people write treefiles without it.
jsonutil: Remove useless GCancellable

Spotted by mbarnes from review of
https://github.com/projectatomic/rpm-ostree/pull/52/commits