docker: change Fedora base image to fedora:27

Using "fedora:latest" makes behavior different depending on when you
actually pulled the image from the docker repository.  In my case,
the supposedly "latest" image was a Fedora 25 download from 8 months
ago, and the new "test-debug" test was failing.

Use "27" to improve reproducibility and make it clear when the image
is obsolete.

Cc: Fam Zheng <famz@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1515755504-21341-1-git-send-email-pbonzini@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

tests/docker/dockerfiles/fedora.docker

-FROM fedora:latest
+FROM fedora:27
 ENV PACKAGES \
     ccache gettext git tar PyYAML sparse flex bison python3 bzip2 hostname \
     glib2-devel pixman-devel zlib-devel SDL-devel libfdt-devel \