doc: Added make file target, treating warnings as errors.

Added htmlstrict make file target, warnings are treated
as errors making sphinx-build stop doc creation.

Change-Id: Ic851ded3b8634559f312ac529dcf5afa624e74b9
Signed-off-by: Giuseppe Vallarelli <giuseppevallarelli@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/11651
Tested-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>