treecompose: Add initramfs-args to treefile

We're building generic initramfs images on the server side, but dracut
has logic to pick up some things from the host, like filesystems.

In the absence of host-specific initramfs images, it needs to be up to
the generating system what kernel modules end up in the initramfs.
Provide a generic option to passthrough dracut arguments.
db list: Fix command line parsing regression

This broke with https://github.com/projectatomic/rpm-ostree/pull/64 I
believe.

The argument parsing here was rather hairy, and I think this patch
clarifies things, in addition to fixing the bug.