core: Remove unnecessary String.format in logging calls

Use slf4j string formatting instead of String.format call in logging.

Change-Id: I1fc06b84903db89386c51a63b1589e0d0e3a8a15
Bug-Url: https://bugzilla.redhat.com/1109871
Signed-off-by: Martin Perina <mperina@redhat.com>