Update gnt-cluster(8) for be/hyp parameter syntax

Now it displays:

--hypervisor-parameters hypervisor:hv-param=value [ ,hv-param=value ... ]
--backend-parameters be-param=value [ ,be-param=value ... ]

Sorry for the super-long lines :( Is there a better way to insert spaces
without pushing them to the resulting man page?

Reviewed-by: iustinp