Remove OstreeConfigUpdater

Stub out origin load for now. It imports gi.repository
that conflicts with gobject, which we use elsewhere.
Fix mismerge of repo config load
wip, refactor file stuff, add config.py, remove repo_file.py
Make OstreeConfig use OstreeConfigFileStore

If OstreeConfig hasn't created a repo_file_store
backing store, create one (a OstreeConfigFileStore)
and save()

Make OstreeConfigFileStore open config file
and create a RepoFile() if it doesn't have one,
and save().

This is to support creating an OstreeConfig
with a backing store, but being able to save
to the default.

Misc name cleanup.
Fix test cases to look for ostree.config

ostree.repo_file is no more.
Package ostree plugin as a sub-package.