compose: Add support for "default_target"

Currently the systemd RPM ships with default.target ->
graphical.target, which is either itself changed by Anaconda (via
parsing /etc/sysconfig/desktop, which...anyways let's stop here).

Or anaconda might set it directly to multi-user.target.

For rpm-ostree, we perform some minimal level of "preconfiguration"
per tree, so they are directly usable without an intervening
installer.

As an example for fedora-atomic/base/core, we just want
multi-user.target.  Thus, this patch provides the treefile author a
declarative mechanism to set it.