docker: Update fedora image to 28

Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20180601023557.9770-1-famz@redhat.com>

tests/docker/dockerfiles/fedora.docker

-FROM fedora:27
+FROM fedora:28
 ENV PACKAGES \
     ccache gettext git tar PyYAML sparse flex bison python3 bzip2 hostname \
     gcc gcc-c++ llvm clang make perl which bc findutils glib2-devel \