Generate coverage HTML reports by default.

Change-Id: I840a379bce93df52e7725b87c6bd6ee95fcdbebd
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Reviewed-on: http://review.gluster.org/5031
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>