Replace %r with '%s' in masterd/instance.py

I still don't know why Michael is a fan of %r, but in the meantime
this patch changes:

WARNING: import u'import-2011-07-29_01_39_33-y3gZKV' on node1 failed:
Exited with status 1

into:

WARNING: import 'import-2011-07-29_01_39_33-y3gZKV' on node1 failed:
Exited with status 1

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>