1120772: Don't traceback on missing /ostree/repo

If /ostree/repo doesn't exist, try to write out
/ostree/repo/config. This avoids tracebacks for the
scenario of running the ostree plugin on a system that
is not a ostree system.
Merge pull request #1047 from candlepin/alikins/1120772_no_ostree_repo

1120772: Don't traceback on missing /ostree/repo