functions: Yet another fix for retnum persistence

There were two cases in which the new retnum variant of persist fails:

1. config file is still bind mounted
2. If everything goes well, then the function returned True
   which got interpreted as a return value of 1.

The target config file is now always unmounted, before a mount is tried,
and the return True  on success got removed.

Change-Id: I2caeea64c79bb55a614cd363082d70afcdd20501
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120650
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>