compose: Move boot-location handling to postprocessing

Originally I was trying to have all of the treefile parsing
in treecompose, and then call other functions as API.  But that
turns out to be unrealistic.  We'll need finer grained control
over this in the future.

Therefore, let's move the boot-location handling down, in preparation
for further commits which parse the treefile in the commit phase.
compose: Add 'etc-group-members'

Currently adding human users to a system group such as 'wheel' does
not work with shadow-utils as it exists now.  This is admittedly
a hack; basically we single out "wheel" as going in /etc/group, via:

    "etc-group-members": ["wheel"],

A more comprehensive solution to this will be:
https://github.com/projectatomic/rpm-ostree/issues/49