RPC 'node_info': <storage_type,key> instead of vg_names

This replaces the field 'vg_names' in the RPC call of 'node info' by
'storage_units'. A storage unit is a tuple <storage_type,key>
and a generalization of a vg_name. The list of vg names is replaced by
a list of storage units. The modified RPC call will be used to report
storage space for more than just lvm volume groups. What the 'key' is
depends on the storage type. For storage type lvm-vg, the key is the
volume group name. To keep backward compatibility, all functions that
use the old vg_names, convert them to a list where every volume group
is mapped to a tuple [('lvm-vg',volume_group)] before making the call.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>