Enable gnt-cluster modify to hv/beparams

This patch enables the cluster modify to change:
  - enabled hypervisor list
  - hvparams (per hypervisor)
  - beparams (only the default group)

Syntax:
  gnt-cluster modify -B vcpus=3 -H xen-pvm:no_initrd_path

Validation for parameters is somewhat missing - the individual
hypervisors will be checked for syntax and validation, but beparams
doesn't have validation yes (nowhere), it should be added here once we
have a global method (will come soon).

Reviewed-by: imsnah