compose: Support "preserve-passwd" option (enabled by default)

The checking code from #56 landed, and started triggering for me on
the `dockerroot` user. It's nice to know it works. Then the issue
is... "what now"?

It turns out in the case of `dockerroot` it's actually unused, so we
could fix this by deleting it. But in general we need to support
dynamic uids/gids/. And we can't yet take a hard dep on #49.

So this patch changes things so we take a copy of the passwd/group
data from the previous commit.  Any users subsequently added in the
*new* commit will be additive.

Closes: https://github.com/projectatomic/rpm-ostree/issues/78