Default empty OstreeRemotes or OstreeCore

If they are not specified via args, make the default
an empty inst of the containers instead of None.
When persisting a OstreeCore copy all items.

OstreeCore is just a dict, and we currently don't
need to know any of it's keys or values, so when
we update or add it to the confif file, just
add all of it key=value into the section.
Test cases for OstreeConfig and ..RepoFileWriter
Remove per class repo_file_store_class

Class can be extend with self_init_store()
Add more tests.

Start testing ostree action_invoker, and rest of
classes in model and config.

Tests are skeletons at the moment, and do little
validation other than 'didnt throw an exception'