core: Replace log4j with slf4j in docs

Replaces log4j with slf4j in docs module.

Change-Id: I3b7ebf1740eb46aa6e03925ca3530ecf1296f5fe
Bug-Url: https://bugzilla.redhat.com/1109871
Signed-off-by: Martin Perina <mperina@redhat.com>