Ensure correct value is logged in config deprecation warning

Fixes: https://bugs.launchpad.net/juju-core/+bug/1394751

When logging a warning about using provisioner-safe-mode, the warning message had the wrong suggested value for the new harvest mode setting.  The logger function now is given what the new value should be so it can use it in the log message.

(Review request: http://reviews.vapour.ws/r/516/)