Skip HasValidVG when --no-lvm-storage on cluster init

This patch does two things:
- Remove "vg_name" from _OP_REQP due to the introduction of
  --no-lvm-storage. Since vg_name option has as default now None and is
  only set to the DEFAULT_VG if lvm_storage is enabled, this is needed
- It changes LUInitCluster.CheckPrereq() to skip _HasValidVG check
  initializing the cluster with --no-lvm-storage. Furthermore it adds to
  the help message the statement of the possibility to use
  --no-lvm-storage if no 'xenvg' volume group is found.

Reviewed-by: iustinp