Add: --sysroot and --os arguments

These match OSTree.  There are a variety of use cases here.  One is
for test suites; we can stand up a temporary sysroot directory, and
operate on content inside there.

Another is doing virtual machine upgrades offline from a host system,
or upgrading a different OS.

The duplication here is a bit unfortunate; if we add a lot more
commands we should revisit this and perhaps have a common option
group.