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
doc/treefile.md: The `repos` are not pulled from the system `/etc/yum.repos.d`
compose: Make 'bootstrap_packages' actually optional now

It has in practice been optional since:
827e711eb74135f075f02f5c44685e4e35c1a8b1

Now let's let people write treefiles without it.
jsonutil: Remove useless GCancellable

Spotted by mbarnes from review of
https://github.com/projectatomic/rpm-ostree/pull/52/commits
Release 2014.111
Delete rpmqa-sorted-and-clean

This is no longer needed now that we use hawkey for this.
treecompose: Suppress signal handling borkage from rpm and librepo
Some initial librepo code
more subs